Ignore libavcodec/tests/mpeg12framerate, a test program

Add to libavcodec/tests/.gitignore an entry for test
program libavcodec/tests/mpeg12framerate . Other
similar test programs, e.g. jpeg2000dwt and dct, are
ignored in a similar way.

On initially checking out master, and doing "./configure"
and "make clean", "git status" reports no untracked
files. After running "make fate", "git status" reports
untracked file "libavcodec/tests/mpeg12framerate".

mpeg12framerate is a unit test program. It was apparently
introduced in commit
278c308ceae6b8d7bac1dfc24518821aae603988, on
Tue Sep 12 22:11:56 2017 +0100. It added a new function
ff_mpeg12_find_best_frame_rate() to
libavcodec/mpeg12framerate.c , and the code in
libavcodec/tests/mpeg12framerate.c to exercise that
function. This commit also added the new program to
the FATE suite, but it omitted a .gitignore entry.

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
