Discriminator tests: stop timing the sixteen-thousand-cookie requests (closes #308)

The two tests that send sixteen thousand cookies checked the answer time
on the wall clock, which a busy host can push past the bound. They no
longer check how long the answer took. The first is renamed to say what
it checks: a 431 before the Core Rule Set reads the request.

TestARefusedRequestDoesNotWaitForASlowGeoJS still checks that a refusal
over the header limits waits on nothing.

Model: opus-5-5

internal/discriminator/answertime_test.go

@@ -67,20 +67,15 @@ func fillHeaders(req *http.Request, total int) {
 	}
 }
 
-func TestSixteenThousandCookiesAreRefusedWellWithinTheTimeLimit(t *testing.T) {
+func TestSixteenThousandCookiesAreRefusedBeforeTheCoreRuleSetReadsThem(t *testing.T) {
 	t.Parallel()
 
 	s := startDiscriminator(t, "", map[string]string{wafMode: block})
 	req := s.question(siteA, client, "/")
 	req.Header.Set(cookieHeader, strings.Repeat("a;", 16000))
 
-	began := time.Now()
 	got := do(t, req)
 
-	if took := time.Since(began); took >= answerTimeLimit/2 {
-		t.Errorf("answered after %s, want well within %s", took, answerTimeLimit)
-	}
-
 	wantDisposition(t, got, tooLarge, discriminator.DispositionDeny)
 
 	line := s.out.requestLine(t)
@@ -209,13 +204,8 @@ func TestObserveModeAllowsARequestOverTheHeaderLimitsUnread(t *testing.T) {
 	req := s.question(siteA, client, "/")
 	req.Header.Set(cookieHeader, strings.Repeat("a;", 16000))
 
-	began := time.Now()
 	got := do(t, req)
 
-	if took := time.Since(began); took >= answerTimeLimit/2 {
-		t.Errorf("answered after %s, want well within %s", took, answerTimeLimit)
-	}
-
 	wantDisposition(t, got, http.StatusOK, discriminator.DispositionAllow)
 
 	line := s.out.requestLine(t)