elements_flvmux.test_incrementing_timestamps is racy

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Test name: check.gst-plugins-good.elements_flvmux.test_incrementing_timestamps
Command: '/builds/jh-hsd/gst-plugins-good/gst-build/build/subprojects/gst-plugins-good/tests/check/elements_flvmux'
=================

Running suite(s): flvmux


Unexpected critical/warning: gst_clock_id_unref: assertion 'id != NULL' failed

Stack trace:
gst_debug_get_stack_trace (gstinfo.c:2788)
gst_check_log_critical_func (gstcheck.c:281)
g_logv (/usr/lib64/libglib-2.0.so.0.5800.1:0x7fab14f436db)
g_log (/usr/lib64/libglib-2.0.so.0.5800.1:0x7fab14f438cf)
test_incrementing_timestamps (flvmux.c:846)
srunner_run_tagged (check_run.c:465)
gst_check_run_suite (gstcheck.c:1067)
main (flvmux.c:907)
__libc_start_main (/usr/lib64/libc-2.28.so:0x7fab14cd340f)
_start (/builds/jh-hsd/gst-plugins-good/gst-build/build/subprojects/gst-plugins-good/tests/check/elements_flvmux:0x4025da)

0%: Checks: 1, Failures: 1, Errors: 0
../subprojects/gstreamer/libs/gst/check/gstcheck.c:286:F:general:test_incrementing_timestamps:0: Unexpected critical/warning: gst_clock_id_unref: assertion 'id != NULL' failed
Check suite flvmux ran in 0.013s (tests failed: 1)

I also got it locally to timeout once, and could relatively easily reproduce the above failure by running the test forever.

CC @hgr who wrote the test :)