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

internal/iplist/iplist_internal_test.go

@@ -646,9 +646,13 @@ func TestRestartLoadsCopiesFromDisk(t *testing.T) {
 	second.Start(t.Context())
 	second.Stop(t.Context())
 
+	// The download takes its time after writing the file, and the restart
+	// reads the file's time. A busy host can put seconds between the two.
+	const maxUpdatedDifference = 15 * time.Second
+
 	listings := lookup(t, second, listedAddr)
 	if len(listings) != 1 ||
-		listings[0].Updated.Sub(downloaded).Abs() > time.Second {
+		listings[0].Updated.Sub(downloaded).Abs() > maxUpdatedDifference {
 		t.Errorf("after the restart, listings %+v, want one updated at %s",
 			listings, downloaded)
 	}