tests/ztest/mock: remove usage of legacy k_fifo_get()

Legacy FIFO operations were failing and thus the TC was failing to run.

Stop using k_fifo_get() for allocation and use a bitmap allocator. A
couple of the bitmap operations should be moved to a common header
once ZEP-1347 is completed.

Passes on all arches and boards, whitelist removed; ARM excluded
though due to missing bitfield implementation as per ZEP-82.

Note there is a false checkpatch positive in the decl of
sys_bitfield_find_first_clear().

Change-Id: I5d43f804d6bec3a464124accbe3be238f9cade82
Signed-off-by: Inaky Perez-Gonzalez <inaky.perez-gonzalez@intel.com>
Signed-off-by: Anas Nashif <anas.nashif@intel.com>