check: add GstTestClock as a deterministic clock for testing

API: GstTestClock
API: gst_test_clock_new()
API: gst_test_clock_new_with_start_time()
API: gst_test_clock_set_time()
API: gst_test_clock_advance_time()

https://bugzilla.gnome.org/show_bug.cgi?id=683012
9 files changed