PHPUnit: Allow developers to use their own custom phpunit.xml file.

The order of precedence of configuration files for PHPUnit is:
 - phpunit.xml (allowing custom user defined configuration)
 - phpunit.xml.dist (configuration distributed with the software)
PHPUnit: Add tests/phpunit.xml to gitignore.
gitignore: Fix typo + adjust comment format.
Merge pull request #3872 from bantu/phpunit-xml-dist

Use phpunit.xml.dist instead of phpunit.xml ...