Updated the test suite dispatcher to allow use of PHPUnit from Composer

Closes #3479
fixes failing test cases in ModelTaskTest if not run in test-suite

Tests 'testSkipTablesAndAll' and 'testExecuteIntoAll' didn't took fixture
table 'core.number_tree' into account.
Merge pull request #3489 from rchavik/2.5-composer

Updated the test suite dispatcher to allow use of PHPUnit from Composer
Merge pull request #3493 from Schlaefer/fix-failingModelTaskTest

fixes failing test cases in ModelTaskTest if not run in test-suite