Rename 'MPEG-4 support' to 'H.263 support'.

I think this is less confusing.

This test actually refers to MPEG-4 Part 2 Visual, the MPEG
extension of H.263. Referring to is as 'MPEG-4' right before
the H.264 test is confusing, since that refers to the container
shared by both profiles as well as the older codec.

I didn't change the internal name of the test because I wasn't sure if that would confuse back-end statistics collection.