Reviewed by Mihai Parparita.

        Fix some potentially flaky tests
        https://bugs.webkit.org/show_bug.cgi?id=56049

        These tests were not resilient to parser yields for reasons explained below.

        * media/video-src-plus-source.html: loadstart or loadmetadata could have fired before
        the waitForEvent() calls. So define them prior to the <video> element.
        * platform/mac/accessibility/aria-expanded-notifications.html: The aria events could
        have fired before successfullyParsed was set, so run the test in the onload handler.
        * plugins/npruntime/npruntime.html: The plugin could have been loaded before
        successfullyParsed was set, so set it before including the plugin.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@80670 268f45cc-cd09-0410-ab3c-d52691b4dbfc