Pass proper dates in tests

This gets rid of the Moment deprecation warning - passing things
into moment() without a specific format will not be supported.
Remove duplicate tests

These tests were asserting exactly the same thing as the test above
them since this functionality was moved, so remove the tests.
Only test the attribute we care about

Don't test the whole HTML.
Merge pull request #876 from alphagov/improve-tests

Improve tests