A lot of test changes have accumulated in the loom repo, this includes both new tests and updates to existing tests. Some of these updates can be brought to the main line, and specifically:

- The existing tests for pinning use synchronized blocks. In preparation for changes to allow carrier thread be released when a virtual threads parks or blocked on monitorenter, the tests are changed to pin by having a native frame on the stack. This includes some test infrastructure to make it easy to add further tests for pinning.

- The test for the JFR jdk.VirtualThreadPinned event is refactored to allow for additional cases where the event may be reported.

- ThreadAPI is expanded to cover more cases of UHE.

- GetStackTraceWhenRunnable is refactored to not use a Selector, this is in preparation for changes that will release the carrier when a selection operation needs to block.

- StressStackOverflow is dialed down to run for 1 instead of 2 mins.

- A few misc. cleanups, such as CarrierThreadWaits changed to use assertEquals rather than assertTrue.