Make the routewatch tests pass on a loaded host (closes #213)

Five prefix handler tests gave HandleMessage two seconds to apply a full
batch of 25,000 prefixes, which under the race detector on a loaded host
takes longer. A test now waits up to stepTimeout, 15 seconds, for each step
it expects.

The slowest tests filled production limits, such as the peering handler's
500,000 paths. The peering handler now takes its path limit, and the prefix
handler its limit on one write of the table's changes, as they take their
timer interval. Those tests give small limits, and the tests of the maximum
batch size lower it to one batch. This also fixes
#215.

The test helpers bound every wait, Stop included, so a test that hangs in
one fails with its own message.

Model: opus-5-5

internal/routewatch/asnfetcher_internal_test.go

@@ -224,7 +224,7 @@ func TestASNFetcherStopReturnsByItsDeadline(t *testing.T) {
 
 	select {
 	case <-lookupStarted:
-	case <-time.After(handleMessageTimeout):
+	case <-time.After(stepTimeout):
 		t.Fatal("the fetcher did not start the lookup")
 	}
 
@@ -240,22 +240,22 @@ func TestASNFetcherStopReturnsByItsDeadline(t *testing.T) {
 
 	select {
 	case <-f.done:
-	case <-time.After(handleMessageTimeout):
+	case <-time.After(stepTimeout):
 		t.Fatal("the fetcher goroutine did not return after the lookup ended")
 	}
 }
 
 // TestRecordKeepsOnlyTheStatsWindow records a success and a failure every
-// second, by a clock the test sets, far longer than statsWindow, and never
+// second, by a clock the test sets, for three times statsWindow, and never
 // asks for the stats. The fetcher must keep only the times within the last
 // statsWindow.
 func TestRecordKeepsOnlyTheStatsWindow(t *testing.T) {
 	t.Parallel()
 
 	const (
-		records = 100_000
 		step    = time.Second
 		want    = int(statsWindow / step)
+		records = 3 * want
 	)
 
 	clock := time.Now()