`ghcide` tests shouldn't use `expectNoMoreDiagnostics`

Example:

https://github.com/haskell/haskell-language-server/blob/b035c21c2b4f31bbc0836648e65e675dc14b1e19/ghcide-test/exe/BootTests.hs#L58

This waits for 5 seconds and then just quits. We don't know whether this actually finished loading, or whether it finished 4.5 seconds ago (realistically, after 2 s).

Rather, we should insert a warning that we can wait for to make sure we know loading has finished. This will make the test more correct and perhaps even faster.

This is related to #4193