Move PHPUnit tests for `behat-tags.php` over

[schlessera]

The unit tests for behat-tags.php cannot be run in the wp-cli/wp-cli framework repo anymore, they need to be run here, where that file actually exists.

PHPUnit also needs to be set up to run in this repository.

• Create tests folder
• Add tests/test-behat-tags.php file (source)
• Create phpunit.xml.dist file