fate: gapless: fix mp3 tests

Seeking to a negative time did not have the desired effect of seeking to
the next valid position (the file start). On the other hand, just
"-ss 0" will normally seek to a position higher than 0, because it adds
the start time of the file. (The start time is not 0 because the gapless
code skips a few samples from the start.)

Fix this by using the "-seek_timestamp 1" option, which makes "-ss 0" do
what you'd expect it would do.

Also put the -ss option at the right place, before -i. This actually
makes it seek, instead of something completely else. The ".out-3" test
is no different in the -usetoc 0/1 cases, because the seeking is
inaccurate (in both cases).

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
