tests/dash_mpd: take account of Period start in expected timestamps

tests/dash_mpd: take account of Period start in expected timestamps

The start of each segment is relative to the Period start, minus
the presentation time offset.

As specified in section 5.3.9.6 of the MPEG DASH specification:
	The value of the @t attribute minus the value of the
	@presentationTimeOffset specifies the MPD start time of
	the first Segment in the series.

Several tests use a Period@start value of 10 seconds, which either
needs to be taken into account when calculating expected timestamps
or have that attribute removed.

This commit uses a mix of updating the timestamps and removing the
start attribute, so that both the case of its presence and absence
is tested.