rproxy tests give a loaded host time before their limits run out (closes #185)

Four rproxy tests set a time limit of a second or less, and each needed a
step done before that limit ran out. On a loaded host the limit ran out
first, rproxy answered as SPEC.md says it should, and the test failed. Each
of these limits is now 3 seconds:

- TestAnAppTooSlowToFinishItsAnswerIsCutOff, which now also checks that the
  visitor got the app's 200 and the start of its answer;
- TestAVisitorTooSlowToTakeTheAnswerIsCutOff;
- TestAVisitorThatStopsSendingGetsAnAnswerRProxyGivesByTheDeadline;
- TestAReportServeWAFLeavesUnansweredIsDroppedAtTheTimeLimit.

Judgement call: tests whose limits already give 2 seconds or more are left as
they are.

Model: opus-5-5

internal/rproxy/rproxy_test.go

@@ -1353,8 +1353,10 @@ func TestAReportServeWAFLeavesUnansweredIsDroppedAtTheTimeLimit(t *testing.T) {
 		})
 
 	app := startApp(t, greet)
+	// The time limit is long enough for a loaded host to answer the question before
+	// it runs out.
 	addr, out := startRProxy(t, backend, app.URL, map[string]string{
-		timeoutSetting: "1000",
+		timeoutSetting: "3000",
 	})
 
 	t.Cleanup(func() { close(release) })