MDL-55944 testing: Reduce sleep usage.

Remove sleep() and replace with waiting for a second to roll over,
this results in a simpler call the guarantee time() has moved forward

lib/phpunit/tests/advanced_test.php

@@ -358,7 +358,7 @@ class core_phpunit_advanced_testcase extends advanced_testcase {
 
         $this->setCurrentTimeStart();
         $this->assertTimeCurrent(time());
-        sleep(2);
+        $this->waitForSecond();
         $this->assertTimeCurrent(time());
         $this->assertTimeCurrent(time()-1);