Resolve memory leaks in tests reported by miri

[chescock]

Objective

Make it possible to run miri with memory leak detection enabled. It is enabled by default, so it reports errors whenever I run it locally. And running it in CI should help prevent any new leaks from being introduced.

See #4310 and #4959 for previous attempts to enable this. Note that miri has added backtraces to leaked allocations since then (rust-lang/rust#109061), so leaks are much easier to diagnose.

Solution

The first leak: When trying to drop non-send data on the wrong thread, the allocation for the value was being leaked. We can't drop the value from the wrong thread, but we could still deallocate its storage.

Rework the thread check in NonSendData::drop so that it still deallocates, setting present = false to ensure the value's drop is not called. Change the panic! to a warn! so that we can have the check work the same way during unwinding.

The second leak: The tests for Interned intentionally leak data! So we can't exactly "fix" them, but I'd like miri to stop failing on them. Then I realized that miri does not report leaks for our use of Interned for things like ScheduleLabel. That's because those values are stored in a static, so are still reachable.

So, add a static to each of the tests with intentional leaks and store the leaked values there. miri will consider those values reachable, and not report any leaks.