tests/fate: Don't keep unnecessary temp files

tests/fate: Don't keep unnecessary temp files

Some FATE tests use files created by other FATE tests as input files;
this mostly affects the seek tests which use files from vsynth_lena as
well as acodec-pcm as input files. In order to make this possible the
temporary files of all the vsynth* and all acodec-pcm tests are kept.
Yet only a fraction of these files are actually used. This commit
changes this to only keep the files that are actually needed for other
tests. This reduces the size of the tests/data/fate folder after a full
FATE run from 2024727441B to 138739312B.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>