Add ability to customize test execution using main wrapper plugin

The jtreg run tests in separate thread. It was proven that it is useful to allow to run tests in virtual thread instead of j.l.Thread as a stress test in project Loom.

This fix is intended to allow to implement custom Main Wrapper which allow user to control execution by implementing CustomMainWrapper interface.

Currently, it is allowed to add additional JVM options and implement
Thread createThread(ThreadGroup tg, Runnable task);
to customize execution of task.