gnlcomposition: add eos_thread_function to replace eos_main_thread.

	+ This is a WIP commit.
gnlcomposition: When removing a ghostpad, remove the probe_handlers associated to its target.
tests: sanitization.

	+ Create a no_install libtestutils.
	+ Remove code from common.h to put it in common.c
	+ Abstract away bus polling from test_simplest # TODO use it as much as possible.
	+ Fix various little errors spotted thanks to new flags.
WIP for playback progression checking.
gnlcomposition: Don't flush_start / flush_stop according to user's flags.

	+ We do it ourselves when needed
	+ Doing so led the pipeline to lose state, which was updating the base time.