Prevent Zendesk from creating tickets for tests

For callback feature tests, tickets were being created on every test
run. This change tries to prevent that. Especially for the remote
BrowserStack tests which run against production.
Using constants in definitions

So that it's clear in the tests and in the model where they are defined.

Amended the features to use constants also
Merge pull request #391 from ministryofjustice/zen-exclude

Prevent Zendesk from creating tickets for tests