Fix JS tests prebuild for clinker

JS tests that should have failed pass on clinker, by looking into clinker output it seems to be due to the `grunt test` task, that was not rebuilding JS files properly

How to reproduce:
- `git co e3e274b && grunt && grunt test` - rebuild all JS fails etc. JS tests pass
- `git co b06b43d && grunt test` - how clinker does it.. tests pass, but should fail(!), since there are 2 failing tests
- `grunt && grunt jasmine` rebuilds JS files and yields the correct, expected results (2 failing tests).

the typical dev workflow is to use `grunt dev` which rebuilds the files on file changes (e.g. a git checkout), which is probably why no-one never noticed this in their local dev environment.

This PR contains the steps to reproduce and verify the fix on clinker too:
- build we found that should not have passed: http://clinker.cartodb.net/jenkins/job/CartoDB-PR-testing/781/console
- build w/ changes from this PR that force a test to fail: http://clinker.cartodb.net/jenkins/job/CartoDB-PR-testing/869/console
- build w/ fix to previously forced test-fail, now works: http://clinker.cartodb.net/jenkins/job/CartoDB-PR-testing/870/console