Tests: fail test runs on deprecations/notices/warnings

As this package is used in the CI pipeline for other packages, it is important for this package to be ready for new PHP releases _early_, so failing the test suite on deprecations/notices and warnings is appropriate.

Apparently PHPUnit 10.0 already included a `failOnWarning` option, while PHPUnit 10.1 brings additional `failOnNotice` and `failOnDeprecation` options.

Unfortunately, these options are problematic for cross-version PHPUnit usage for the following reasons:
1. They do not translate to the same behaviour between PHPUnit < 10 and PHPUnit 10.x.
    In PHPUnit 9.x, the `convertDeprecationsToExceptions` and sister-options would only act on PHP native deprecations/notices/warnings and would cause a failed build with a non-zero exit code when set to `true` and any PHP deprecations/notices/warnings would be found.
    In PHPUnit 10.x, the `failOnWarning` and sister-options act on **_all_** deprecations/notices/warnings, not just the PHP native ones, and cause a failed build with a non-zero exit code when set to `true`.
    In practice, this means that when the `failOn*` options are used, PHPUnit native deprecations/notices/warnings will now also fail a build, which was not the case in PHPUnit < 10.
2. As the XML file is validated against the XSD file, this means that you cannot add the `failOnNotice`/`failOnDeprecation` options in a configuration file which will be used on PHPUnit 10.0 as that would fail the XSD validation, leading to a PHPUnit warning which would fail the build.
3. Along the same lines, as the `include`/`exclude` elements within the `coverage` element are deprecated as of PHPUnit 10.1, in favour of using `source` with `include`/`exclude`, a configuration which validates on PHPUnit 10.0 will not validate correctly on PHPUnit 10.1 leading to a deprecation notice, which would fail the build when `failOnDeprecation` is on.
... etc...

This basically means that if you want PHPUnit to fail a build on PHP native deprecations/notices/warnings (which you should want for open source projects, most definitely open source projects in the PHP tool chain), you will now no longer be allowed to have **_any_** PHPUnit deprecations/notices/warnings as those will now also fail the build.

It also means that a new release of PHPUnit can randomly start failing builds if a change was made in PHPUnit which causes a new deprecation/notice/warning to be thrown.

All in all, not pretty and it makes things a heck of a lot less stable, but it is what it is for now, so we better deal with it.

So, to get round this and to have the most optimal configuration for PHPUnit 10.x possible at this time....
1. The `failOnWarning` option is added to the `phpunit10.xml.dist` file as it is already available in PHPUnit 10.0.
2. A new "Migrate configuration" step needs to be added to the GH Actions test workflow.
    This step will migrate the `phpunit10.xml.dist` file, which is compatible with PHPUnit 10.0, to a version which is compatible with whichever PHPUnit 10.x version the tests are being run on.
    This step will fail when there is nothing to migrate, so it needs a `continue-on-error` to ignore that failure.
    Note: the migration step will also fail when the original XML file doesn't validate against the original XSD, which is why the base configuration needs to be for PHPUnit 10.0.
3. As for the `failOnNotice`/`failOnDeprecation` options: those cannot be added to the config for the above mentioned reasons (failure to validate against the PHPUnit 10.0 XSD), but they also cannot be added to the "default" command used for running the tests on PHPUnit 10 as a run against PHPUnit 10.0 would then fail with an "Unknown option ...." error.
    In other words, the only realistic way to add these, for now, is to add them to the command on the fly for those test runs where PHPUnit 10.1 or higher will be used.

Refs:
* sebastianbergmann/phpunit 5196
* sebastianbergmann/phpunit@fb6673f

.github/workflows/test.yml

@@ -103,6 +103,9 @@ jobs:
 
     continue-on-error: ${{ matrix.experimental }}
 
+    env:
+      EXTRA_PHPUNIT_CLIARGS: '--fail-on-deprecation --fail-on-notice'
+
     steps:
       - name: Checkout code
         uses: actions/checkout@v3
@@ -150,17 +153,24 @@ jobs:
         if: ${{ matrix.coverage == true && ! startsWith( steps.phpunit_version.outputs.VERSION, '10.' ) }}
         run: composer coverage
 
+        # Migrate PHPUnit configuration to deal with changes in the coverage/source setting across PHPUnit 10.x
+        # versions as otherwise the warning about these would fail the build (which to me, feels like a bug).
+      - name: "Migrate configuration (PHPUnit 10.0+)"
+        continue-on-error: true
+        if: ${{ startsWith( steps.phpunit_version.outputs.VERSION, '10.' ) && steps.phpunit_version.outputs.VERSION != '10.0' }}
+        run: vendor/bin/phpunit -c phpunit10.xml.dist --migrate-configuration
+
       - name: "Run the unit tests (PHPUnit 10.0+)"
         if: ${{ matrix.coverage == false && startsWith( steps.phpunit_version.outputs.VERSION, '10.' ) }}
         # Don't fail the build on a test run failure against a future PHPUnit version.
         continue-on-error: ${{ matrix.phpunit == 'dev-main' }}
-        run: composer test10
+        run: composer test10 -- ${{ steps.phpunit_version.outputs.VERSION != '10.0' && env.EXTRA_PHPUNIT_CLIARGS || '' }}
 
       - name: "Run the unit tests with code coverage (PHPUnit 10.0+)"
         if: ${{ matrix.coverage == true && startsWith( steps.phpunit_version.outputs.VERSION, '10.' ) }}
         # Don't fail the build on a test run failure against a future PHPUnit version.
         continue-on-error: ${{ matrix.phpunit == 'dev-main' }}
-        run: composer coverage10
+        run: composer coverage10 -- ${{ steps.phpunit_version.outputs.VERSION != '10.0' && env.EXTRA_PHPUNIT_CLIARGS || '' }}
 
       # PHP Coveralls doesn't fully support PHP 8.x yet, so switch the PHP version.
       - name: Switch to PHP 7.4

phpunit10.xml.dist

@@ -13,6 +13,7 @@
     displayDetailsOnTestsThatTriggerWarnings="true"
     displayDetailsOnTestsThatTriggerNotices="true"
     displayDetailsOnTestsThatTriggerDeprecations="true"
+    failOnWarning="true"
     >
 
     <testsuites>