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 <[email protected]>

Author: aescolar

tests/subsys/testsuite/fff_fake_contexts/testcase.yaml

@@ -12,6 +12,5 @@ tests:
     tags: test_framework
     platform_allow:
       - qemu_cortex_m3
-      - native_posix/native/64
       - native_sim/native/64
       - native_sim