rtpjitterbuffer: various test-improvements

Mainly generalize all the latest tests that have found various stalls in the jitterbuffer, so that they only consist of a series of packets with various seqnum/rtptime/rtx combinations, arriving at a specific time.

This means future tests can be more easily written to prove certain behavior does not cause stalls.

Also fix the warning on windows: warning C4244: 'initializing': conversion from 'double' to 'gint', possible loss of data