PHPUnit: Move the config file

[jrfnl]

Summary

This PR can be summarized in the following changelog entry:

• N/A

Relevant technical choices:

If phpunit.xml or phpunit.xml.dist (in that order) exist in the current working directory and --configuration is not used, the configuration will be automatically read from that file.

From: https://phpunit.de/manual/6.5/en/organizing-tests.html#organizing-tests.xml-configuration

In other words, moving the phpunit.xml.dist file to the project root allows for running PHPUnit without passing --configuration and makes it more obvious that there are unit tests associated with the PHP code in this plugin.

Includes adding phpunit.xml to the .gitignore file to allow devs to use that file for their own custom configuration.

Test instructions

This PR can be tested by following these steps:

• Check the Travis output to verify that the unit tests are still run correctly (compare against a previous build on develop).
• Try running the unit tests from the root of the repository using the composer test command.

[Dieterrr]

CR Done 👍