gh-104584: Clean up and fix uops tests and fix crash (#106492)

The uops test wasn't testing anything by default,
and was failing when run with -Xuops.

Made the two executor-related context managers global,
so TestUops can use them (notably `with temporary_optimizer(opt)`).

Made clear_executor() a little more thorough.

Fixed a crash upon finalizing a uop optimizer,
by adding a `tp_dealloc` handler.