modules: Add fff mocking framework

Issue #38643

Introduce a more powerful and well established mocking framework
into Zephyr. It also allows running the actual FFF tests using the
zephyr SDK and ztest framework to ensure compatibility.

As per TSC meeting, the fff.h header was directly added to
subsys/testsuite/include/. As per the guidelines, the file is exactly
the same as it is in FFF's library, but re-styled with clang-format.

The west.yml entry was added using the "ci" group and filtered by
default. (note that the tests will break until the CI actually
specifies that the group is needed).

Signed-off-by: Yuval Peress <peress@google.com>