watcher: save and notify nothing for a cut-short port or TLS check (closes #185)

When shutdown cancels a check that is under way, the rest of the check
still runs with the cancelled context. The resolver already drops a
lookup the context cut short, but a cancelled connection attempt was
saved as a closed port or a failed certificate check and notified as
Port Change or TLS Failure. The watcher now drops a port or TLS check
result when its context was cancelled, the same way. The test runs a
check with the context already cancelled, using the real resolver and
the real port and TLS checkers; no query is sent and no connection is
made.

Model: opus-5-5

README.md

@@ -618,7 +618,8 @@ repository's `Dockerfile` and runs it. The app needs:
    abandoned, and the number abandoned is logged at warn level rather
    than dropped silently. Notifications generated after shutdown has
    begun are refused and logged, so a late burst cannot extend the
-   shutdown.
+   shutdown. A DNS lookup, port check or TLS check that shutdown cuts
+   short saves nothing and sends no notification.
 
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