LeAudio: Fix flaky unit tests

Since the audio context notifications on the other thread invalidate the
existing audio set configurations. We should wait of rthe main loop to
finish the job before we start to use these configurations.

We are not consistent in every test with the synchronisation after the
injected notification. This patch moves the synchronisation to a
notification sendind helper function, eliminating the need for new tests
to call the synchronisation routine manually.

Bug: 302775890
Test: atest --host UnicastTest#StartStream_AvailableContextTypeNotifiedLater --rerun-until-failure=1000
Change-Id: I38a892c829779616ac20b0dbb283dcef5204c4e0