[Bug 1030905] fjord feedback api tests for dates

Test the get api when filtering by date.
Add the following tests:
* test_date_start
* test_date_end
* test_date_delta_with_date_end
* test_date_delta_with_date_start
* test_date_delta_with_date_end_and_date_start
* test_date_delta_with_no_constraints
And the following helper method:
* create_test_data
* _test
Merge pull request #319 from iankronquist/1030905-more-api-tests

1030905 more api tests