Bug#28718 Running backup testcase fails in mysql testsuite of MySQL-enterprise-5.0.40

- Fix test to work on OS where backslashes are forward

mysql-test/include/uses_vardir.inc

@@ -7,9 +7,8 @@
 # outside of it's vardir anyway
 #
 
-let $datadir=
-  query_get_value("SHOW VARIABLES LIKE 'datadir'", Value, 1);
-if (`select LOCATE("$MYSQLTEST_VARDIR", "$datadir") != 1`)
+
+if (`select LOCATE('$MYSQLTEST_VARDIR', REPLACE(@@datadir, '\\\\', '/')) != 1`)
 {
   skip Need mysqld in MYSQLTEST_VARDIR;
 }