mtr: move if(unix_socket) test to include/have_unix_socket.inc

mysql-test/include/have_unix_socket.inc

+--source include/not_embedded.inc
+
+if (!$AUTH_SOCKET_SO) {
+  skip No unix_socket plugin;
+}
+
+if (!$USER) {
+  skip USER variable is undefined;
+}
+

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

---source include/not_embedded.inc
-
-if (!$AUTH_SOCKET_SO) {
-  skip No auth_socket plugin;
-}
+--source include/have_unix_socket.inc
 
 let $plugindir=`SELECT @@global.plugin_dir`;