Build/Test Tools: Configure logging for PHPUnit and multisite.

This ensures a log file is created after running the PHPUnit test suite as a multisite.

In practice, this file is used by the Host Test Runner to collect a list of failing tests to submit to wordpress.org.

See WordPress/phpunit-test-runner#267.

Props rishabhwp, nimeshatxecurify.
Fixes #63884.

git-svn-id: https://develop.svn.wordpress.org/trunk@60781 602fd350-edb4-49c9-b593-d223f7449a82

Author: desrosj

tests/phpunit/multisite.xml

@@ -73,4 +73,7 @@
 			</exclude>
 		</whitelist>
 	</filter>
+	<logging>
+		<log type="junit" target="tests/phpunit/build/logs/junit.xml" />
+	</logging>
 </phpunit>