update seek regressions for r9917

(correct initial timestamps which have AV_NOPTS_VALUE)

sorry, i will rm tests/seek_test ; make ; make seektest from now on

Before r9917 lavf replaced the initial dts which were unknown (AV_NOPTS_VALUE)
by values starting from -frame_duration or another guessed value.
After r9917 lavf buffers packets until it finds a known timestamp or it buffered
too much, it will then correct the previous unknown timestamps in the buffer exactly.
For the seek tests that meant that a few initial timestamps would get changed.
If no packet in the whole stream contains a timestamp then all timestamps
will be changed by first_frame_duration or so as 0 is used instead of
-frame_duration as startpoint ...

Originally committed as revision 9949 to svn://svn.ffmpeg.org/ffmpeg/trunk