CI/QA: make the phpunit.xml.dist file cross-version compatible
CI/QA: make the phpunit.xml.dist file cross-version compatible

The specs for the `phpunit.xml.dist` file have changed numerous times over the years with new options being added and outdated options being removed.
These specs are contained in a schema XSD file for each minor PHPUnit version.

This commit:
* Adds a proper XML file header;
* Annotates that an XSD schema is available and used;
* Adds the `backupGlobals` directive.
    The default value for the top-level `backupGlobals` directive has [changed in PHPunit 6 from `true` to `false`](https://phpunit.de/announcements/phpunit-6.html). To maintain the same behaviour cross-version, this directive is now explicitly set in the config file.
* Adds a `name` to the testsuite.
    As of PHPUnit 7, each testsuite is required to have a `name` attribute. Prior to that version, the `name` attribute was already allowed, so adding the attribute improves cross-version compatibility of the config file.
* Fixes the indentation to tabs.

What I have not changed:
* The `testsuite` node was not allowed according to the XSD until PHPUnit 4.4, however as the config with the `testsuite` works just fine in PHPUnit 3.x and 4.x, this is not an issue.

Yoast/whip