app/appengine: fix test breakage from CL 210977

Forgot Trybots. Yay me.

Change-Id: Ic18237bb976337febf7bb75f1e86374632dc2447
Reviewed-on: https://go-review.googlesource.com/c/build/+/210980
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
diff --git a/app/appengine/dash.go b/app/appengine/dash.go
index 7d35931..f9f8245 100644
--- a/app/appengine/dash.go
+++ b/app/appengine/dash.go
@@ -22,10 +22,12 @@
 
 var (
 	maintnerClient  = createMaintnerClient()
-	datastoreClient = createDatastoreClient()
+	datastoreClient *datastore.Client // not done at init as createDatastoreClient fails under test environments
 )
 
 func main() {
+	datastoreClient = createDatastoreClient()
+
 	// authenticated handlers
 	handleFunc("/clear-results", AuthHandler(clearResultsHandler)) // called by x/build/cmd/retrybuilds
 	handleFunc("/result", AuthHandler(resultHandler))              // called by coordinator after build