selftests/seccomp: Add SKIP for failed unshare()

Running the seccomp tests under the kernel with "defconfig"
shouldn't fail. Because the CONFIG_USER_NS is not supported
in "defconfig". Skipping this case instead of failing it is
better.

Signed-off-by: Yang Guang <yang.guang5@zte.com.cn>
Signed-off-by: David Yang <davidcomponentone@gmail.com>
Reviewed-by: Shuah Khan <skhan@linuxfoundation.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/7f7687696a5c0a2d040a24474616e945c7cf2bb5.1648599460.git.yang.guang5@zte.com.cn