Fix an intermittent failure with TestWebappClassLoaderExecutorMemoryLeak.
If the executor threads did not stop quickly enough, Thread.stop() was used. Sometimes this resulted in the executor not shutting down and the test failing. This makes using clearReferencesStopThreads marginally safer.

git-svn-id: https://svn.apache.org/repos/asf/tomcat/trunk@1530213 13f79535-47bb-0310-9956-ffa450edef68
Some more false positives

git-svn-id: https://svn.apache.org/repos/asf/tomcat/trunk@1530219 13f79535-47bb-0310-9956-ffa450edef68