tests/subsys/testsuite/fff_fake_contexts: Don't test on native_posix

native_posix is now deprecated.
Building these tests in both native_sim and native_posix does not
improve coverage for the tests or subsystem but doubles CI time.
As anyhow native_posix will be removed all together in
2 releases, let's remove it already for these.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>