Fix 'gr-diff-comment' test when run multiple times

The test worked fine on CI since they are run with a clean profile.
When tests were run in a normal browser profile manually, tests
'draft creation/cancelation' and 'draft saving/editing' failed
alternately because of local storage use for saving draft comments.

Since these tests do not test local storage (yet), we can just create
a stub for 'gr-storage#getDraftComment' which simulates a clean state.

Change-Id: Ibb63be4f1c395d47e0022f63d0d2ba0a7c87b137
diff --git a/polygerrit-ui/app/elements/diff/gr-diff-comment/gr-diff-comment_test.html b/polygerrit-ui/app/elements/diff/gr-diff-comment/gr-diff-comment_test.html
index 6c27a36..d2262ab 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff-comment/gr-diff-comment_test.html
+++ b/polygerrit-ui/app/elements/diff/gr-diff-comment/gr-diff-comment_test.html
@@ -120,6 +120,9 @@
           return Promise.resolve({ok: true});
         },
       });
+      stub('gr-storage', {
+        getDraftComment: function() { return null; },
+      });
       element = fixture('draft');
       element.changeNum = 42;
       element.patchNum = 1;
@@ -263,6 +266,7 @@
               'Element should be enabled when done updating draft.');
           assert.equal(draft.message, 'saved!');
           assert.isFalse(element.editing);
+          fireStub.restore();
           done();
         });
       });