8268598: mark hotspot runtime/stringtable tests which ignore external VM flags

Reviewed-by: hseigel, mseledtsov

Author: iignatev

test/hotspot/jtreg/runtime/stringtable/StringTableVerifyTest.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2018, 2020, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2018, 2021, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -25,6 +25,7 @@
  * @test
  * @bug 8199137
  * @summary VerifyStringTableAtExit should not crash
+ * @requires vm.flagless
  * @library /test/lib
  * @modules java.base/jdk.internal.misc
  * @run driver StringTableVerifyTest