ffprobe: Fix fate tests for ffprobe in cases where TARGET_PATH differs from the curre...

ffprobe: Fix fate tests for ffprobe in cases where TARGET_PATH differs from the current path

In these cases, we must pass the full path of the file to ffprobe
(as the current working dir on the remote system, e.g. when invoked
with "ssh remote ffprobe ..." isn't the wanted one).

The input filename passed to ffprobe is also included in the output,
which is part of the reference test data. Add a new option to
ffprobe to allow overriding what path is printed, to keep the
original relative path in the tests.

An alternative approach could be an option to allow requesting omitting
the file name from the dumped data, and updating the test references
accordingly.

Signed-off-by: Martin Storsjö <martin@martin.st>