don't run unix_socket tests when $USER is already present in mysql.user

(as it's done in 10.0)

mysql-test/include/have_unix_socket.inc

@@ -8,3 +8,6 @@ if (!$USER) {
   skip USER variable is undefined;
 }
 
+if (`SELECT count(*) <> 0 FROM mysql.user WHERE user = '$USER'`) {
+  skip \$USER=$USER which exists in mysql.user;
+}