Rename libav.regression.ref --> lavf.regression.ref.

This matches the new name of the test target.

Originally committed as revision 18229 to svn://svn.ffmpeg.org/ffmpeg/trunk

diff --git a/tests/libav.regression.ref b/tests/lavf.regression.ref
similarity index 100%
rename from tests/libav.regression.ref
rename to tests/lavf.regression.ref