scanning: add tests for failure cases of GetNextImage, StartScan

Add new unit tests to cover the cases where the kGetNextImageMethod or
kStartScan D-Bus calls encounter:
* null responses
* responses without valid protos
* responses claiming the scan failed

Bug: b:169573779
Change-Id: I47c19e91229ee7f0e1995b03cdeec28d20cd92ce
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2518448
Commit-Queue: Paul Moy <pmoy@chromium.org>
Reviewed-by: Jesse Schettler <jschettler@chromium.org>
Cr-Commit-Position: refs/heads/master@{#824509}

chromeos/dbus/lorgnette_manager/lorgnette_manager_client.cc

@@ -321,7 +321,7 @@ class LorgnetteManagerClientImpl : public LorgnetteManagerClient {
     GetNextImage(response_proto.scan_uuid());
   }
 
-  // Callend when a response to a GetNextImage request is received from
+  // Called when a response to a GetNextImage request is received from
   // lorgnette. Handles stopping the scan if the request failed.
   void OnGetNextImageResponse(std::string uuid, dbus::Response* response) {
     DCHECK_CALLED_ON_VALID_SEQUENCE(sequence_checker_);