test case for JCasHashMap multi-threaded not reliable

There is a test in the JCasHashMapTest which runs multiple threads, and depends on sub threads running ahead of the main thread, but it uses sleep delays to attempt to achieve that. This is not reliable, in case the whole Java process is pre-empted for longer than the longer sleep. Change this to use, in addition, thread priorities - making the main thread have a lower priority than the sub-threads, so if the wait times are both expired, still the sub-thread runs before the main test thread.