This PR fixes several causes of test flakiness and re-enables the tests
that were disabled in #10665, #10669 and #10673.
Specifically, it fixes:
- A race condition in the file worker that caused it to report an
incomplete snapshot prefix in the inlay hint request (confirmed to be
the cause of #10665)
- A bug in the test runner where it didn't correctly account for
non-deterministic message ordering inducing different RPC pointer
numbering (confirmed to be the cause of #10673)
- A race condition in the watchdog that would sometimes cause the module
hierarchy to be empty (likely the cause of #10669, but not confirmed as
this issue only reproduced again once in tens of thousands of test runs
on various machines, including CI)
- An unrelated bug in the module hierarchy implementation that would
cause it to report an empty module hierarchy when the file was changed
It also replaces some calls to `Task.get` in the language server with
`IO.wait` to protect the code against unfortunate compiler re-ordering.
80b8e44072