Fix test failure in plugins.unix_socket when running tests as user root.

(The problem is that if $USER is root, then the authentication will log
in with the mysqld user root. And then CURRENT_USER() returns a valid
user, giving .result file difference).

mysql-test/suite/plugins/t/unix_socket.test

@@ -1,4 +1,7 @@
 --source include/not_embedded.inc
+# If we run this as root, $USER gets authenticated as the `root' user, and we
+# get .result differences from CURRENT_USER().
+--source include/not_as_root.inc
 
 if (!$AUTH_SOCKET_SO) {
   skip No auth_socket plugin;