MWDebugTest: clear log before running tests

Some other test could have added their own logs, for example
deprecated notice. In that case, the first test will fail because
the log array already has some content :-b

tests/phpunit/includes/debug/MWDebugTest.php

@@ -10,9 +10,6 @@ class MWDebugTest extends MediaWikiTestCase {
 			MWDebug::init();
 			$MWDebugEnabled = true;
 		}
-	}
-
-	function tearDown() {
 		/** Clear log before each test */
 		MWDebug::clearLog();
 	}