gd: Sort out flag dependencies and fix advertising tests

Make sure that gd unit test is using bluetooth_flags_c_lib_for_test.
Without this patch both bluetooth_flags_c_lib and
bluetooth_flags_c_lib_for_test was compiled into the test binary.

Also this fixes advertising tests.

Bug: 356594223
Flag: TEST_ONLY
Test: atest bluetooth_test_gd_unit
Change-Id: I37c992ee03bb249cfdcc343fa6e7bd29d26e3e68