test: hlsenc: Make the hls_fmp4 sample file name match the target

test: hlsenc: Make the hls_fmp4 sample file name match the target

Previously, with the file name not matching the target, the files
were regenerated every time fate is rerun - contrary to the other
test targets in the same file. (While regenerating it every time
might be desireable, as that's what the test is about, the file
at least has a dependency on the ffmpeg executable, making them
regenerated every time the executable is updated - and this change
at least makes it consistent with the rest.)

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