tests: flvmux: Instead of using the testclock, just send eos event for drain
When using the testclock for determining clock in test, it is sometimes observed that the clock entry is not registered in time by the aggregator, then deadlock occurs between the aggregator and the test thread.
In mylocal environment as well, the following test passed:
GST_CHECKS=test_rollover_timestamps meson test -C build --suite gst-plugins-good elements_flvmux --repeat 10000
Fixes: #704 (closed)