tests/subsys/testsuite/fff_fake_contexts: Enable for native_sim

Enable this test which runs in native_posix_64 in native_sim,
Switch from native_posix_64 to native_sim_64 as default test platform

Signed-off-by: Alberto Escolar Piedras <[email protected]>

Author: aescolar

tests/subsys/testsuite/fff_fake_contexts/testcase.yaml

@@ -8,8 +8,10 @@ tests:
   testing.testsuite.fff_fake_contexts:
     integration_platforms:
       - qemu_cortex_m3
-      - native_posix_64
+      - native_sim_64
     tags: test_framework
     platform_allow:
       - qemu_cortex_m3
       - native_posix_64
+      - native_sim_64
+      - native_sim