Scrollbars look different on different platforms, so must be made invisible in tests. As a drive-by, xdescribe was broken with the new test runner.
References #2524
This patch introduces a goldentest.js. The tests inside
the file should rely on "golden" results rather then asserts.
For now, goldentest.js allows only image expectations. If the
actual result doesn't match the expected result, the two files
are created under `test/output` folder:
- The '-actual.png' contains the actual test result
- The '-diff.png' contains the diff between images
