selftests: skip seccomp get_metadata test if not real root

The get_metadata() test requires real root, so let's skip it if we're not
real root.

Note that I used XFAIL here because that's what the test does later if
CONFIG_CHEKCKPOINT_RESTORE happens to not be enabled. After looking at the
code, there doesn't seem to be a nice way to skip tests defined as TEST(),
since there's no return code (I tried exit(KSFT_SKIP), but that didn't work
either...). So let's do it this way to be consistent, and easier to fix
when someone comes along and fixes it.

Signed-off-by: Tycho Andersen <tycho@tycho.ws>
Acked-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Shuah Khan <shuah@kernel.org>