JFR: Split up TestThreadStartEndEvents.java

The test test/jdk/jdk/jfr/event/TestThreadStartEndEvents.java tests both the jdk.ThreadStart and the jdk.ThreadEnd event. It would be better to have the event testst in separate files. If one event fails, no need to put both on both on problem list. The ThreadStart event now has a stack trace, which the ThreadEnd event lacks, so verification is different.