unit_test: add cmake macros for mocking files

When using the mocked version of a file,
the real source needs to be removed and replaced
with the mocked sources. For certain files such as
fwk_notification some build flags need to be enabled
for proper function. Macros are provided to bundle the
required steps together.

Signed-off-by: Brett Warren <brett.warren@arm.com>
Change-Id: If625b39baed2f03b07d6b0589844ef0df2bb2d95