flvmux: test_video_caps_late occasionally timeouts

Happened after ~10k individual runs, but happens more frequently when its run as part of the whole testsuite.

Happens as of 8b155d71. Fedora 29.

Reproducible with the following from a gst-build checkout:

./gst-uninstalled.py gst-validate-launcher check.gst-plugins-good.elements_flvmux.test_video_caps_late --n-runs 10000 -fs --mute --no-display --dump-on-failure --meson-no-rebuild

Output

check.gst-plugins-good.elements_flvmux.test_video_caps_late: Failed 'Application returned 1'
       You can reproduce with: GST_REGISTRY='/home/alatiera/centricular/gst-build/build/subprojects/gst-plugins-good/tests/check/elements_flvmux.registry' GST_STATE_IGNORE_ELEMENTS='aasink autoaudiosrc autoaudiosink autovideosrc
 autovideosink cacasink cairotextoverlay jackaudiosrc jackaudiosink
 osssrc osssink osxaudiosink osxaudiosrc osxvideosrc osxvideosink
 pulsesink pulsesrc pulsemixer v4l2src' GST_PLUGIN_SYSTEM_PATH_1_0='' DISPLAY=':0' CK_DEFAULT_TIMEOUT='20' GST_CHECKS='test_video_caps_late' GST_PLUGIN_LOADING_WHITELIST='gstreamer:gst-plugins-base:gst-plugins-good@/home/alatiera/centricular/gst-build/build' GST_PLUGIN_PATH_1_0='/home/alatiera/centricular/gst-build/build' /home/alatiera/centricular/gst-build/build/subprojects/gst-plugins-good/tests/check/elements_flvmux

Dumping log files on failure

Dumping contents of /home/alatiera/centricular/gst-build/validate-output/logs/check/gst-plugins-good/elements_flvmux/test_video_caps_late

=================
Test name: check.gst-plugins-good.elements_flvmux.test_video_caps_late
Command: '/home/alatiera/centricular/gst-build/build/subprojects/gst-plugins-good/tests/check/elements_flvmux'
=================

Running suite(s): flvmux
0%: Checks: 1, Failures: 0, Errors: 1
../subprojects/gst-plugins-good/tests/check/elements/flvmux.c:527:E:general:test_video_caps_late:0: (after this point) Test timeout expired
Check suite flvmux ran in 20.001s (tests failed: 1)