rtpjitterbuffer: add failing test that reproduces a stall

So what happens here is that at some point we receive packet #10508,
which triggers the big gap logic, placing that buffer inside the
gap_packets GList:
gst_rtp_jitter_buffer_chain: expected #10529, got #10508, gap of -21
handle_big_gap_buffer: buffer too old -21 < -20, first one - waiting

Then packet #10550 arrives:
gst_rtp_jitter_buffer_chain: expected #10530, got #10550, gap of 20

This causes too many timers to be pending, causing a reset:
gst_rtp_jitter_buffer_chain: 182 pending timers > 180 - resetting
gst_rtp_jitter_buffer_reset:< flush and reset jitterbuffer

Inside the reset() code, the next_seqnum is set to the first buffer
in gap_packets, #10508:
gst_rtp_jitter_buffer_reset: setting next_seqnum to #10508

And reset() calls chain() with all its gap_packets, pushing it out:
gst_rtp_jitter_buffer_chain: Received packet #10508 at time 123:45:05.364650711, discont 0, rtx 1

However, #10508 is actually an RTX packet, so it gets dropped:
gst_rtp_jitter_buffer_chain:<rtpjitterbuffer0> Unsolicited RTX packet #10508 detected, dropping

Next #10550 is pushed from reset():
gst_rtp_jitter_buffer_chain:<rtpjitterbuffer0> Received packet #10550 at time 123:45:05.516136868, discont 0, rtx 0

But because next_seqnum is set to #10508, we get this:
handle_next_buffer: Sequence number GAP detected: expected 10508 instead of 10550 (42 missing)

So now the jitterbuffer is completely stalled, all buffer arriving now will
just queue up until it sees the promised #10508, which will be ~65000
packets from now, and we will trigger the "full" logic before then,
causing a deadlock.
27 jobs for !550 with hgr/jb-stall-take-2 in 76 minutes and 56 seconds (queued for 3 seconds)
detached
Status Job ID Name Coverage
  Preparation
passed #2128156
gst indent

00:00:30

passed #2128155
manifest

00:00:26

 
  Build
passed #2128166
gstreamer
build cerbero cross-android universal

00:24:22

passed #2128170
gst-ios-13.2
build cerbero cross-ios universal

01:12:30

passed #2128167
gstreamer
build cerbero cross win32

00:15:52

passed #2128168
gstreamer
build cerbero cross win64

01:08:50

passed #2128165
gstreamer
build cerbero fedora x86_64

00:12:16

passed #2128169
gst-macos-10.15
build cerbero macos x86_64

00:51:31

passed #2128161
build clang fedora x86_64

00:05:24

passed #2128157
build fedora x86_64

00:04:51

manual #2128164
docker windows 1809 gstreamer-windows allowed to fail manual
build msys2
passed #2128158
build nodebug fedora x86_64

00:03:26

passed #2128159
build static fedora x86_64

00:12:21

passed #2128160
build static nodebug fedora x86_64

00:03:22

passed #2128162
docker windows 1809 gstreamer-windows
build vs2017 amd64

00:11:43

manual #2128163
docker windows 1809 gstreamer-windows allowed to fail manual
build vs2017 x86
 
  Test
failed #2128171
gstreamer
check fedora

00:03:52

passed #2128172
gstreamer
integration testsuites fedora 1/4

00:13:49

passed #2128173
gstreamer
integration testsuites fedora 2/4

00:13:00

passed #2128174
gstreamer
integration testsuites fedora 3/4

00:11:13

passed #2128175
gstreamer
integration testsuites fedora 4/4

00:12:50

passed #2128177
gstreamer
valgrind bad

00:59:33

passed #2128178
gstreamer
valgrind ges

00:04:33

failed #2128176
gstreamer
valgrind good

01:00:00

 
  Integrate
passed #2128179
cross-android universal examples

00:06:26

passed #2128181
gst-ios-13.2
cross-ios universal examples

00:03:53

passed #2128180
documentation

00:03:52

 
Name Stage Failure
failed
valgrind good Test The script exceeded the maximum execution time set for the job
|█████████████████████████████████████████████████-------------------------------| [455/742]
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|███████████████████████████████████████████████████-----------------------------| [474/742]

|████████████████████████████████████████████████████----------------------------| [483/742]

|█████████████████████████████████████████████████████---------------------------| [492/742]

|██████████████████████████████████████████████████████--------------------------| [501/742]

|███████████████████████████████████████████████████████-------------------------| [511/742]

Pulling docker image gitlab/gitlab-runner-helper:x86_64-577f813d ...
ERROR: Job failed: execution took longer than 1h0m0s seconds
failed
check fedora Test
           Total: 3311
Uploading artifacts...
gst-build/build/meson-logs/: found 2 matching files
validate-logs: found 3732 matching files

Uploading artifacts to coordinator... ok
id=2128171 responseStatus=201 Created token=JA4f-Zwv
Uploading artifacts...
validate-logs/*.xml: found 1 matching files

Uploading artifacts to coordinator... ok
id=2128171 responseStatus=201 Created token=JA4f-Zwv
ERROR: Job failed: exit code 1