test: hlsenc: Use unique init/segment file names for the fmp4_ac3 test

test: hlsenc: Use unique init/segment file names for the fmp4_ac3 test

Previously, the hls-fmp4 and hls-fmp4_ac3 tests used the same file
names for init and segment files, which occasionally could cause
corruption and failed tests, if the input files for both tests are
generated in parallel, as they could overwrite each other.

This happened to work some of the time, as the fmp4_ac3 test actually
only checked the init segment file (which the fmp4 test case never
wrote, due to using the incorrect hls_segment_type option) and the
fmp4 test case always regenerated the input files due to mismatched
target and file names.

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