audiorate, GstAudioEncoder: Add unit test when receiving SEGMENT event

When the source is restarted or replaced, it generates a new SEGMENT
event that caused audiorate and audio encoders to have a discontinuity
in their output PTS/DTS even when the source itself does not have any
discontinuity.

Do this in opusenc tests because that was the original use-case. Also
noticed those tests were not even built before.