selftests/vm/pkeys: Add a regression test for setting PKRU through ptrace

This tests PTRACE_SETREGSET with NT_X86_XSTATE modifying PKRU directly and
removing the PKRU bit from XSTATE_BV.

Signed-off-by: Kyle Huey <me@kylehuey.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Link: https://lore.kernel.org/all/20221115230932.7126-7-khuey%40kylehuey.com