We have test to make sure that certain configurations do print
warnings. However, the normal operation of the muxer within this
test always printed a warning, so those tests to check for
extra warnings didn't essentially guard anything.

The warning that always was printed, "track 1: codec frame size is
not set" was not present in the libav fork where this testcase
originated, it was removed in f234e8a3.

Set the frame size for the audio stream to silence the warning,
and use this frame size in a couple later calculations, and check
that one test configuration doesn't print warnings.

Setting the frame size apparently changes the rounding of a timestamp
in the ismv muxing testcase.
Signed-off-by: Martin Storsjö <martin@martin.st>