composition: Cleanup and fix commit introducing our own thread

Update the pipeline from the streaming thread before emiting EOS
gnlcomposition: When removing a ghostpad, remove the probe_handlers associated to its target.
gnlcomposition: remove unused pending_idle private member
composition: Rename "eos thread" to "update pipeline thread"
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.