leaudio: Make sure all unit tests depeds on flags are tested

This patch removes all the TEST_F_WITH_FLAG test cases and configures
flag properly for given tests.

It uses bluetooth_flags_c_lib_for_test for that.

Bug: 331775328
Flag: TEST_ONLY
Test: atest bluetooth_le_audio_test bluetooth_le_audio_client_test
bluetooth_test_broadcaster

Change-Id: I338fceea491dd7dad35dc7e6a8c9faf7758fdd1b