test(hack): load the strict-mode helper in fork-new-images.bats (#4805)

## What this PR does

The bats unit lane is red on main, and so is the "Unit & controller
tests" check on pull requests built on it. `hack/bats-strict-setup.bats`
requires each unit file to load `test_helper`, which turns `set -u` back
on for its tests. `hack/fork-new-images.bats` is the only unit file
without that line, so the audit fails with `strict-mode setup missing or
overridden`. This PR adds the line. All six tests in that file pass
under `set -u`.

### Screenshots

Not a UI change.

### Downstream repositories

- [x] No downstream repository is affected by this change
- [ ] [cozystack/website](https://github.com/cozystack/website) -
follow-up:
- [ ]
[cozystack/terraform-provider-cozystack](https://github.com/cozystack/terraform-provider-cozystack)
- follow-up:
- [ ]
[cozystack/ansible-cozystack](https://github.com/cozystack/ansible-cozystack)
- follow-up:
- [ ] [cozystack/ccp](https://github.com/cozystack/ccp) - follow-up:
- [ ] [cozystack/talm](https://github.com/cozystack/talm) - follow-up:
- [ ] [cozystack/cozyhr](https://github.com/cozystack/cozyhr) -
follow-up:
- [ ] [cozystack/cozy-proxy](https://github.com/cozystack/cozy-proxy) -
follow-up:
- [ ]
[cozystack/cozystack-telemetry-server](https://github.com/cozystack/cozystack-telemetry-server)
- follow-up:
- [ ]
[cozystack/external-apps-example](https://github.com/cozystack/external-apps-example)
- follow-up:
- [ ] [cozystack/examples](https://github.com/cozystack/examples) -
follow-up:
- [ ] [cozystack/community](https://github.com/cozystack/community) -
follow-up:

### Release note

```release-note
test(hack): fork-new-images.bats loads the strict-mode helper, so the strict-mode audit in the bats unit lane passes again.
```


<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **Tests**
* Updated the fork-image tests to load their shared test setup before
running, ensuring the tests have the required support in place.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

hack/fork-new-images.bats

@@ -4,6 +4,8 @@
 # e2e-fork.yaml uses for the push allowlist, or the early check and the push
 # disagree about what counts as new.
 
+load test_helper
+
 ROOT="$(pwd)"
 SCRIPT="$ROOT/hack/fork-new-images.sh"
 FORK_WF="$ROOT/.github/workflows/e2e-fork.yaml"