Adds a project-specific phpunit.xml file.

[jhedstrom]

• Fixes #108

This still needs work, as I'm forgetting how/when db credentials are available. We need to properly populate this bit of the file:

    <!-- Example SIMPLETEST_DB value: mysql://username:password@localhost/databasename#table_prefix -->
    <env name="SIMPLETEST_DB" value=""/>

[grayside]

This seems like a decent plan, I'd like to test this out and coordinate with generator-outrigger-drupal before merging.

There should also be test coverage to at least confirm generation of the phpunit.xml.dist file on generator runs.