added XSD to PHPUnit configuration
minor #11329 add XSD to PHPUnit configuration (craue)

This PR was merged into the 2.3 branch.

Discussion
----------

add XSD to PHPUnit configuration

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | --
| License       | MIT
| Doc PR        | --

The syntax check functionality has been removed in PHPUnit 3.6 already. But there's no Composer constraint for PHPUnit, so you can never know which version will actually be used to run tests. Let me know what you think.

Commits
-------

84b5581 added XSD to PHPUnit configuration