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 runs on a fake clock, so a stall on the
host cannot use up its idle timeout.

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

internal/blocklist/blocklist_internal_test.go

@@ -301,9 +301,13 @@ 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)
 	}