CRUDControllerTest: added test for editAction

| Q             | A
| ------------- | ---
| Bug fix?      | yes (flash messages should not be added in ajax request)
| New feature?  | no
| BC breaks?    | yes (added `supportsPreviewMode` to the AdminInterface)
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -