Wait for HLS to index before hovering an identifier from base

[thomasjm]

hovers putStrLn fails intermittently with No hover returned. — on main today, not just on
branches. hovers foo in the same file never fails: foo is a local binding, putStrLn is from
base and has no hover until HLS has indexed dependencies. The test hovers immediately after
openDoc, so it races that.

Wrapped in waitUntil, which retries until the assertions hold.

I looked at waiting on work-done progress ($/progress, which lsp-test already tracks via
getIncompleteProgressSessions) instead. It isn't sound: an empty progress set doesn't distinguish
"finished" from "between tasks", so the check can land in the gap before the server starts more
work. Retrying the assertion is the thing that actually converges.

Compiles; the flake itself only reproduces under CI load.