Flaky: SSHHostTests.closingAHostWithdrawsItsQuestion is timing-dependent

Failed on PR #254's macOS CI (unrelated change), alongside the #234 flakes. Added in Phase D (#252). The test appears to depend on timing when a closed SSH host withdraws its pending question. Make it deterministic (await the withdrawal explicitly rather than relying on scheduling).

test(macos): wait on conditions instead of wall-clock budgets