selftests/mm: skip pkey_sighandler_tests if support is missing

selftests/mm: skip pkey_sighandler_tests if support is missing

The pkey_sighandler_tests are bound to fail if either the kernel or CPU
doesn't support pkeys.  Skip the tests if pkeys support is missing.

Link: https://lkml.kernel.org/r/20241209095019.1732120-14-kevin.brodsky@arm.com
Signed-off-by: Kevin Brodsky <kevin.brodsky@arm.com>
Cc: Aruna Ramakrishna <aruna.ramakrishna@oracle.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Joey Gouly <joey.gouly@arm.com>
Cc: Keith Lucas <keith.lucas@oracle.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>