tests_: Add tests for unicast signalling

Test that the new utility functions in libcroesor work correctly; and
add an integration test for end-to-end unicast signalling.

Note that this requires the teardown check in integration.py to be
removed as it was failing. I have not investigated this yet; disabling
it for now is fairly harmless, but in the long run it should be
investigated to see whether it’s possible to describe the system’s
behaviour down to individual main context iterations, or whether that’s
not worth pursuing.

Reviewed-by: Simon McVittie <simon.mcvittie@collabora.co.uk>
Signed-off-by: Philip Withnall <philip.withnall@collabora.co.uk>
Differential Revision: https://phabricator.apertis.org/D3507