Build/Test Tools: Add schema reference to PHPUnit config files.

The current config files validate against the PHPUnit XSD schema for config files for PHPUnit 5.7 – 9.2.

The schema was changed in PHPUnit 9.3, and the `filter` and `logging` settings were deprecated in favor of `coverage` and a different format for `logging`.

This commit explicitly sets the schema against which the files currently validate, for clarity.

Props jrf.
See #53363.

git-svn-id: https://develop.svn.wordpress.org/trunk@51583 602fd350-edb4-49c9-b593-d223f7449a82

Author: SergeyBiryukov

phpunit.xml.dist

@@ -1,4 +1,6 @@
 <phpunit
+		xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+		xsi:noNamespaceSchemaLocation="http://schema.phpunit.de/9.2/phpunit.xsd"
 		bootstrap="tests/phpunit/includes/bootstrap.php"
 		backupGlobals="false"
 		colors="true"

tests/phpunit/multisite.xml

@@ -1,4 +1,6 @@
 <phpunit
+		xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+		xsi:noNamespaceSchemaLocation="http://schema.phpunit.de/9.2/phpunit.xsd"
 		bootstrap="includes/bootstrap.php"
 		backupGlobals="false"
 		colors="true"