[app][mmutest] Enable run_nx test.

Use a thread with exit-on-panic set to test running code out of a
ARCH_MMU_FLAG_PERM_NO_EXECUTE allocation. Also replace test that
allocates without testing with the inverse test instead.

Bug: 147698516
Change-Id: I13ccc420ce9e096705bffd3724d64bf1f58cd211
1 file changed