tests: Bluetooth: Audio: Remove dependency on host mocks

Removes the dependency on the host mocks for LE Audio tests.
This is done by copying the missing mocks for assert into
the audio mocks, and then always including ztest.h.

The inclusion of ztest.h is due to the fact that
arch/cpu.h does not have an appropriate header file for
ztest, and some header files depend on ARCH_STACK_PTR_ALIGN
which is usually defined by those headers in cpu.h. However
ztest.h does define this value as well, and thus needs
to be included always.

Signed-off-by: Emil Gydesen <emil.gydesen@nordicsemi.no>