Creating vsynth_lena.yuv needs the FATE suite,
yet several tests in ffmpeg.mak without a dependency
on samples used it as input file. Fix this by using
vsynth1.yuv (which does not have such a dependency)
instead.
Also use vsynth1.yuv in fate-shortest to avoid
the samples dependency in this test, too.

Fixes ticket #10947.
Reviewed-by: Anton Khirnov <anton@khirnov.net>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>