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

Tests that wait on the real clock failed when the host stalled for a few seconds, although the code was right. Waits for something that should happen are now mostly 15 seconds. Tests whose result depends on a short idle timeout or a production request limit run under `synctest.Test`, on a fake clock, with a fake transport where a real connection would stop that clock. Test code only.

Judgement call: the 5-second waits in three `internal/streamer` tests also became 15 seconds.

Model: opus-5-5

cmd/asinfo-gen/main_test.go

@@ -12,7 +12,7 @@ import (
 
 // parseTimeLimit is how long a test waits for parseCSV to return before it
 // fails.
-const parseTimeLimit = 2 * time.Second
+const parseTimeLimit = 15 * time.Second
 
 // TestParseCSVReadsTodaysLayout parses an as.csv with the country code column
 // that the source added after the description. It expects each AS with its