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>

tools/testing/selftests/seccomp/config

 CONFIG_SECCOMP=y
 CONFIG_SECCOMP_FILTER=y
+CONFIG_USER_NS=y

tools/testing/selftests/seccomp/seccomp_bpf.c

@@ -3444,7 +3444,10 @@ TEST(user_notification_child_pid_ns)
 	struct seccomp_notif req = {};
 	struct seccomp_notif_resp resp = {};
 
-	ASSERT_EQ(unshare(CLONE_NEWUSER | CLONE_NEWPID), 0);
+	ASSERT_EQ(unshare(CLONE_NEWUSER | CLONE_NEWPID), 0) {
+		if (errno == EINVAL)
+			SKIP(return, "kernel missing CLONE_NEWUSER support");
+	};
 
 	listener = user_trap_syscall(__NR_getppid,
 				     SECCOMP_FILTER_FLAG_NEW_LISTENER);
@@ -3509,7 +3512,10 @@ TEST(user_notification_sibling_pid_ns)
 	}
 
 	/* Create the sibling ns, and sibling in it. */
-	ASSERT_EQ(unshare(CLONE_NEWPID), 0);
+	ASSERT_EQ(unshare(CLONE_NEWPID), 0) {
+		if (errno == EPERM)
+			SKIP(return, "CLONE_NEWPID requires CAP_SYS_ADMIN");
+	}
 	ASSERT_EQ(errno, 0);
 
 	pid2 = fork();