glean main thread from ClassPrepareEvent

Author: plummercj

test/hotspot/jtreg/vmTestbase/nsk/jdi/EventSet/suspendPolicy/suspendpolicy001.java

@@ -250,14 +250,15 @@ private void testRun()
 
         ClassPrepareEvent event = (ClassPrepareEvent) eventIterator.next();
         debuggeeClass = event.referenceType();
+        debuggee.setMainThread(event.thread());
 
         if (!debuggeeClass.name().equals(debuggeeName))
            throw new JDITestRuntimeException("** Unexpected ClassName for ClassPrepareEvent **");
         log2("      received: ClassPrepareEvent for debuggeeClass");
 
         log2("......setting up ClassPrepareEvent for breakpointForCommunication");
 
-        ThreadReference mainThread = debuggee.threadByNameOrThrow("main");
+        ThreadReference mainThread = debuggee.mainThread();
         setupBreakpointForCommunication(debuggeeClass);
 
     //------------------------------------------------------  testing section