[coil] change "dummy" to "stub"

A few references are left behind: where we run an autotest that is
still called "dummy".

BUG=b:187793358
TEST=CQ passes

Change-Id: I98165c31088b0c7e9cfe170f2228e851a21975c7
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/3526871
Tested-by: Mike Frysinger <vapier@chromium.org>
Auto-Submit: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Ram Chandrasekar <rchandrasekar@google.com>
Commit-Queue: Ram Chandrasekar <rchandrasekar@google.com>
diff --git a/scripts/test_image_unittest.py b/scripts/test_image_unittest.py
index f7ece60..d84ba8d 100644
--- a/scripts/test_image_unittest.py
+++ b/scripts/test_image_unittest.py
@@ -77,7 +77,7 @@
     """Verify the CWD is in a temp directory."""
 
     class CwdTest(image_test_lib.ImageTestCase):
-      """A dummy test class to verify current working directory."""
+      """A stub test class to verify current working directory."""
 
       _expected_dir = None
 
@@ -111,7 +111,7 @@
     """Verify that "--board", "--test_results_root" are passed to the tests."""
 
     class AttributeTest(image_test_lib.ImageTestCase):
-      """Dummy test class to hold board and directory."""
+      """Stub test class to hold board and directory."""
 
       def testOkay(self):
         pass