LeAudio: Fix flaky unit tests

When a flag gets reset while there are still some callbacks checking
the flag on the main loop thread, the test could crash.
To improve the test separation it is also better to reset flags before
each test, rather than depending on the previous test resetting the
flag during the tear down.

Bug: 380042533
Test: atest bluetooth-test-audio-hal-le-audio-software bluetooth-test-audio-hal-aidl-leaudio-utils bluetooth_le_audio_test bluetooth_le_audio_client_test  bluetooth_test_broadcaster bluetooth_test_broadcaster_state_machine bluetooth_le_audio_codec_manager_test net_test_btif net_test_btif_hh net_test_btif_stack bluetooth_vc_test net_test_eatt
Flag: TEST_ONLY
Change-Id: Iebc81f3f2cd6c6b7579f3a6037dc8f0dc590f4e6