Useful to add or update fate tests.

TARGET_EXEC allows running FATE on simulators, remote targets and
memory checkers. Also document a known issue with Wine and mixed
Unix/Windows line ending.

FATE_SAMPLES is now used directly by the Makefiles, which induces the test
system to run a test with the value of the environment variable as name.
Renaming the environment variable to LIBAV_SAMPLES avoids this problem.

Signed-off-by: Diego Elio Pettenò <flameeyes@flameeyes.eu>
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>

Summarize the information provided in the wiki and the one provided
in fate.txt