Build: Correct the build flag for test service

Make the inclusion of IPC test service conditionally by
'TFM_PARTITION_TEST_CORE_IPC'.

Change-Id: I475ae39b5e43b5ab8a6b3b6f9ebabc9f91f8c853
Signed-off-by: Edison Ai <edison.ai@arm.com>
1 file changed