MDL-71036 phpunit: Remove custom autoloader

Custom autoloaders are deprecated with PHPUnit 9 and will be removed
with PHPUnit 10.

Since PHPUnit 8.5 custom autoloaders don't do much because that
version removed the ability to launch unit tests by class name
and that's exactly the reason we had a custom autoloader (to map
class names to files within our tests). See MDL-67673 about
when direct use of classes was deprecated (8.5), now removed (9.5).

So, as far as it's unused, removing it now, test still can be
selectively using any of:

- a relative path to file (although there are some restrictions comming
  with PHPUnit 9, see sebastianbergmann/phpunit#4105
- using --filter, to point to any classname[::method]
- using --testsuite to run a complete suite
- using --config to point to custom components.

Also, commented out the lib/ajax/tests directory because it doesn't
exist / is empty and PHPUnit 9 emits error when a configured test
directory does not exist. See
sebastianbergmann/phpunit#4493.

Alternative was to completely remove the configuration line, but
decided to keep it around in case some day we want to add some
test there.

lib/phpunit/bootstrap.php

@@ -50,7 +50,6 @@
 
 require_once(__DIR__.'/bootstraplib.php');
 require_once(__DIR__.'/../testing/lib.php');
-require_once(__DIR__.'/classes/autoloader.php');
 
 if (isset($_SERVER['REMOTE_ADDR'])) {
     phpunit_bootstrap_error(1, 'Unit tests can be executed only from command line!');

phpunit.xml.dist

@@ -16,7 +16,6 @@
         stopOnSkipped="false"
         beStrictAboutTestsThatDoNotTestAnything="false"
         beStrictAboutOutputDuringTests="true"
-        testSuiteLoaderClass="phpunit_autoloader"
         >
 
     <php>
@@ -45,7 +44,7 @@
         </testsuite>
         <testsuite name="core_testsuite">
             <directory suffix="_test.php">lib/tests</directory>
-            <directory suffix="_test.php">lib/ajax/tests</directory>
+            <!-- <directory suffix="_test.php">lib/ajax/tests</directory> -->
         </testsuite>
         <testsuite name="core_favourites_testsuite">
             <directory suffix="_test.php">favourites/tests</directory>