acme: in TestFetchCertCancel, do not reply before the cancellation propagates

Fixes golang/go#44611

Change-Id: I81b3c3b7be25fe4e492695fa5935e70aa7b96c07
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/378074
Trust: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Roland Shoemaker <roland@golang.org>

acme/acme_test.go

@@ -975,6 +975,7 @@ func TestFetchCertRetry(t *testing.T) {
 
 func TestFetchCertCancel(t *testing.T) {
 	ts := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		<-r.Context().Done()
 		w.Header().Set("Retry-After", "0")
 		w.WriteHeader(http.StatusBadRequest)
 	}))