test: Gracefully handle running within user namespace with single user

Unprivileged users often make themselves root by unsharing a user namespace
and then mapping their current user to root which does not require privileges.
Let's make sure our tests don't fail in such an environment by adding checks
where required to see if we're not running in a user namespace with only a
single user.

src/test/test-socket-util.c

@@ -170,7 +170,7 @@ TEST(getpeercred_getpeergroups) {
                 struct ucred ucred;
                 int pair[2] = EBADF_PAIR;
 
-                if (geteuid() == 0) {
+                if (geteuid() == 0 && !userns_has_single_user()) {
                         test_uid = 1;
                         test_gid = 2;
                         test_gids = (gid_t*) gids;