tests: Don't build disabled plugins' check tests

https://bugzilla.gnome.org/show_bug.cgi?id=723502
rtpjitterbuffer: Remove raw comparisons of RTP sequence numbers

Several conditional statements perform comparison on RTP sequence
numbers without taking the sequence number rollover into account.
Instead, use the gst_rtp_buffer_compare_seqnum function to perform the
comparison.

https://bugzilla.gnome.org/show_bug.cgi?id=725159
rtpjitterbuffer: Fix uninitialized variable compiler warning