selftests/seccomp: Add SKIPs for failed unshare()

Running the seccomp tests as a regular user shouldn't just fail tests
that require CAP_SYS_ADMIN (for getting a PID namespace). Instead,
detect those cases and SKIP them. Additionally, gracefully SKIP missing
CONFIG_USER_NS (and add to "config" since we'd prefer to actually test
this case).

Signed-off-by: Kees Cook <keescook@chromium.org>