tests: remove queues introducing raciness in flvmux test

Fixes:

GST_CHECKS=test_incrementing_timestamps meson test elements_flvmux \
-C build --repeat 10000

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

What I observed is that between the call to
test_clock_wait_next_pending_id and test_clock_process_next_id,
the clock entry was unscheduled by gst_aggregator_pad_chain_internal.

Removing the queues before flvmux preserve the intention of the test,
but makes it deterministic by making sure buffers are pushed on the
test thread.

I can also observe a SIGSEGV every once in a while but it doesn't
seem related.
6 jobs from flvmux_racy_test in 27 minutes and 54 seconds (queued for 3 seconds)
latest
Status Job ID Name Coverage
  Preparation
passed #63018
gst indent

00:00:28

passed #63017
manifest

00:00:22

 
  Build
passed #63019
build fedora x86_64

00:03:18

 
  Test
passed #63020
check fedora

00:14:59

 
  Full Builds
passed #63021
build android arm64 api28

00:02:51

passed #63022
build cerbero fedora x86_64

00:09:07