watcher: warn of an expiring certificate on every TLS check (closes #204)

An expiry warning was skipped when the last one for that hostname and
address was sent less than DNSWATCHER_TLS_INTERVAL ago. Each TLS check
runs after a DNS pass of varying length, so two checks can be less than
the interval apart, and a certificate about to expire was warned about on
every check or every other check, at random. TLS checks already start
once per interval, so the in-memory record of when each warning was sent
is removed and every check warns, as the README says.

The test that expected the second check to stay silent is replaced by one
that runs TLS checks on state built in the test, with no DNS.

Model: opus-5-5

TODO.md

@@ -19,6 +19,8 @@ trial run of the finished image: https://git.eeqj.de/sneak/dnswatcher/issues/149
 
 # Completed Steps
 
+- 2026-10-01: a certificate within the expiry warning period is warned about on
+  every TLS check, where some checks used to skip it at random (closes #204).
 - 2026-10-01: a domain's NS set is its delegation from the parent zone's
   servers, not whichever of its own servers answered first (closes #200).
 - 2026-10-01: README has Getting Started, Rationale and TODO sections, and its