test: fix test-loopback failure when lacking privileges

Setting up the loopback might fail due to lack of privileges, as it
happens when running unit tests in the Noble CI environment. Skip
the test when it happens.

1584s  862/1330 systemd:test / test-loopback   FAIL  0.01s   exit status 1
1584s /* test_loopback_setup */
1584s Failed to configure loopback network device, ignoring: Operation not permitted
1584s loopback: Operation not permitted

src/test/test-loopback.c

@@ -19,6 +19,8 @@ TEST_RET(loopback_setup) {
         }
 
         r = loopback_setup();
+        if (ERRNO_IS_NEG_PRIVILEGE(r))
+                return log_tests_skipped("lacking privileges");
         if (r < 0)
                 return log_error_errno(r, "loopback: %m");