internal: fix flaky context test

Change-Id: I877fbcfdde33559baabbda2c275164dd98256892
Reviewed-on: https://go-review.googlesource.com/85196
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

internal/token_test.go

@@ -94,14 +94,16 @@ func TestRetrieveTokenWithContexts(t *testing.T) {
 		t.Errorf("RetrieveToken (with background context) = %v; want no error", err)
 	}
 
-	ctx, cancelfunc := context.WithCancel(context.Background())
-
+	retrieved := make(chan struct{})
 	cancellingts := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
-		cancelfunc()
+		<-retrieved
 	}))
 	defer cancellingts.Close()
 
+	ctx, cancel := context.WithCancel(context.Background())
+	cancel()
 	_, err = RetrieveToken(ctx, clientID, "", cancellingts.URL, url.Values{})
+	close(retrieved)
 	if err == nil {
 		t.Errorf("RetrieveToken (with cancelled context) = nil; want error")
 	}