payload_unittest: fix test that used new alias value

BUG=None
TEST=./run_tests

Change-Id: I11c0a8218384f57832b494e928d61c94511c14c4
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/2570092
Reviewed-by: George Engelbrecht <engeg@google.com>
Tested-by: George Engelbrecht <engeg@google.com>
diff --git a/service/payload_unittest.py b/service/payload_unittest.py
index 701b05a..a794818 100644
--- a/service/payload_unittest.py
+++ b/service/payload_unittest.py
@@ -92,5 +92,5 @@
   def testImageType(self):
     """Test _ImageTypeToStr works."""
     # pylint: disable=protected-access
-    self.assertEqual('base', payload._ImageTypeToStr(1))
+    self.assertEqual('image_type_base', payload._ImageTypeToStr(1))
     # pylint: enable=protected-access