Make the tests pass on a busy host (closes #240)

Tests that waited a few seconds for something to happen now wait 15 seconds.
The wait ends as soon as the condition holds, so a passing run takes no longer.

TestStreamNotDroppedWhileLinesArrive, the tests whose requests pass a 2- or
4-second limit in the server, and TestStreamDurationsCountSeconds run on a fake
clock, so a stall on the host cannot change their result.

The checkpoint tests give the writing connection a 30-second busy timeout, so
their limit can be 15 seconds and a checkpoint that waits for a read still
fails them.

Model: opus-5-5

TODO.md

@@ -53,6 +53,9 @@ problem the run finds.
 
 # Completed Steps
 
+- 2026-11-10: the tests no longer fail when a busy host stalls for several
+  seconds. Most such waits are now 15 seconds. The test of the stream's idle
+  timeout runs on a fake clock (closes #240)
 - 2026-11-10: each result of the IP lookup has a `flags` object with `is_tor`
   and `is_bogon`. The public IP lists are named `ip_lists` in the answer,
   `internal/iplist` in the code and `iplists` in the state directory. A list