Pregenerate bugfix.

Missing a call to xbuddy.Get. Broke trying to pregenerate an update
payload to a remotely downloaded image.

BUG=chromium:260491
TEST=manual
start_devserver --image=xbuddy:remote/parrot/latest --pregenerate_update
TEST=update a unittest

Change-Id: I17da3bac6eb60efda0c05738b95e3ea2dcdc7eb9
Reviewed-on: https://gerrit.chromium.org/gerrit/66529
Reviewed-by: Joy Chen <joychen@chromium.org>
Tested-by: Joy Chen <joychen@chromium.org>
Reviewed-by: Chris Sosa <sosa@chromium.org>
Commit-Queue: Joy Chen <joychen@chromium.org>
diff --git a/autoupdate_unittest.py b/autoupdate_unittest.py
index 2917fd2..36bc562 100755
--- a/autoupdate_unittest.py
+++ b/autoupdate_unittest.py
@@ -194,6 +194,9 @@
     au_mock = self._DummyAutoupdateConstructor()
     au_mock.forced_image = "xbuddy:b/v/a"
 
+    self._xbuddy._GetArtifact(
+        ['b', 'v', 'a'], None).AndReturn(('label', constants.TEST_IMAGE_FILE))
+
     au_mock.GetUpdateForLabel(
         autoupdate.FORCED_UPDATE, 'b/v/a').AndReturn('p')
     self.mox.ReplayAll()