notify: make the shutdown tests fail with the right message (closes #116)

drainSlack stood for three things: the deadline given to a drain that
should finish early, the watchdog on a drain that should time out, and
the wait for a delivery to reach the test server. It is now three
constants, each commented with what it bounds and why it is 2s; no
value changed. The idle-drain failure printed that deadline instead of
idleDrainBound, the bound it checks. The cancelled-context test now
also requires the drain to return within idleDrainBound and to log its
debug line, so a drain that logs nothing no longer passes;
newLoggingService records debug level for this.

Model: opus-5-5

TODO.md

@@ -19,6 +19,8 @@ Rationale, Design, TODO, License, Author) if any are still missing.
 
 # Completed Steps
 
+- 2026-10-01: notify shutdown tests use one timing constant per meaning, name
+  the bound they check, and require the drain's debug line (closes #116).
 - 2026-09-29: the live-DNS test package is renamed `internal/livednstest` and
   added to the `test-support` `deny` list in `.golangci.yml`, so `make lint`
   fails when program code imports it (closes #164).