Deprecate old test suite loader #4105

[realFlowControl]

This pull requests belongs to #4105, deprecates the old test suite loader and gives deprecation warnings about:

• multiple test classes in a single file
• test case class name does not match filename

[realFlowControl]

Hey @sebastianbergmann,
the deprecation message is a bit, let's say explicit, but I think its good to help a bit in such a case, what do you think?

$ ./phpunit tests/_files/OneClassPerFile/wrongClassName/
PHPUnit 9.0-gb87137081 by Sebastian Bergmann and contributors.

Runtime:       PHP 7.3.14 with Xdebug 2.9.2
Configuration: /home/bowman/Work/flowcontrol/phpunit/phpunit.xml

Warning: Test case class not matching filename is deprecated
         in /home/bowman/Work/flowcontrol/phpunit/tests/_files/OneClassPerFile/wrongClassName/WrongClassNameTest.php
         class name was 'WrongClassNameBar', expected 'WrongClassNameTest', see #4105

.                                                                   1 / 1 (100%)

Time: 48 ms, Memory: 6.00 MB

OK (1 test, 1 assertion)

/Flo

[sebastianbergmann]

Merged manually, thanks.