[PATCH] pg_test_fsync: close Windows alarm thread handles

Attached is a follow-up to Sehrope's CF7218 timer fix - close each
created alarm-thread handle. Closing the handle leaves the callback
running.

This is a dependent supplement. Apply the included CreateThread
failure-check prerequisite, then this patch, on master 8e82d6610e7.
The prerequisite credits Sehrope; it contains only his timer fix.

Source-extracted probes on native ARM64 and emulated x64 show 200
retained handles before the change and none afterward, with all 200
callbacks completing. The actual pg_test_fsync -s 1 benchmark sequence
also passes on both targets in the combined integration builds.
Formatting and independent reviews pass.

The measurements establish handle cleanup and callback completion;
they aren't performance benchmarks.

Let me know your thoughts.

Best,

Harrison