SPM: Tidying FF-A Memory Sharing tests

Moving code about memory sharing tests to comply with recent changes:
- Placed helper functions that can be used by TFTF and Cactus in files
spm_common.c/h.
- Removed 'ffa_memory_sharing_test' and added its body to handler of
test command CACTUS_MEM_SEND_CMD.

Signed-off-by: J-Alves <joao.alves@arm.com>
Change-Id: I58af9b475730d171306581741becbb0665e1859d
8 files changed