Test when the muted IDL attribute is updated from the content attribute

Blink/WebKit has a bug here and fails the last two parser tests.
Test setting video.muted

video_muted_check.html is made redundant.
Test cloneNode- and innerHTML-created <video muted>

Presto fails the cloneNode test but nothing else.
Test more cases of setting video.muted
Test setting audio.muted

audio_muted_check.html is made redundant.
Test muted for document.write-created elements
Disable the cloneNode muted test pending spec bug resolution
Merge pull request #729 from foolip/muted

Test when the muted IDL attribute is updated from the content attribute