minor #6092 Updated information about testing code coverage. (roga)

This PR was merged into the 2.3 branch.

Discussion
----------

Updated information about testing code coverage.

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets | -

This is just a rebase of #5929 to make it mergeable.

Commits
-------

8a5a239 Updated information about testing code coverage.

create_framework/unit_testing.rst

@@ -26,6 +26,11 @@ using `PHPUnit`_. Create a PHPUnit configuration file in
                 <directory>./tests</directory>
             </testsuite>
         </testsuites>
+        <filter>
+          <whitelist processUncoveredFilesFromWhitelist="true">
+            <directory suffix=".php">./src</directory>
+          </whitelist>
+        </filter>
     </phpunit>
 
 This configuration defines sensible defaults for most PHPUnit settings; more
@@ -180,6 +185,12 @@ Open ``example.com/cov/src/Simplex/Framework.php.html`` in a browser and check
 that all the lines for the Framework class are green (it means that they have
 been visited when the tests were executed).
 
+Alternatively you can output the result directly to the console:
+
+.. code-block:: bash
+
+    $ phpunit --coverage-text
+
 Thanks to the simple object-oriented code that we have written so far, we have
 been able to write unit-tests to cover all possible use cases of our
 framework; test doubles ensured that we were actually testing our code and not