Fix: Clean up phpunit.xml.dist

[localheinz]

Q	A
Bug fix?	yes
New feature?	no
BC breaks?	no
Deprecations?	no
Related tickets	n/a
Documentation	n/a
License	MIT

What's in this PR?

This PR

• references phpunit.xsd in phpunit.xml.dist
• removes an invalid attribute syntaxCheck
• merges duplicate php nodes

Why?

Running

$ composer install && composer test

on current master yields

PHPUnit 7.4.5 by Sebastian Bergmann and contributors.

  Warning - The configuration file did not pass validation!
  The following problems have been detected:

  Line 8:
  - Element 'phpunit', attribute 'syntaxCheck': The attribute 'syntaxCheck' is not allowed.

  Line 33:
  - Element 'php': This element is not expected.

  Test results may not be as expected.

[dbu]

thanks, keep them coming!

[localheinz]

Thank you, @dbu!