Add test case for dev master config

What does this MR do and why?

Closes Follow-up from "Duplicate incident labels array... (#1411 - closed).

The bug that was fixed in !2453 (merged) originated from the fact that in some of the configs, such as stable-branch and dev-master-branch, a constant was used directly in the incident_labels method under the config class. The labels are being updated during triage, and we cannot modify a constant and that was the error we saw. This only happens to configurations that allow auto triage and directly use a constant in the incident_labels method. dev-master-branch is one of these scenarios.

After I adding the test and reverting the fix in !2453 (merged), I can see the test fails so this now properly covers the scenario.

Expected impact & dry-runs

These are strongly recommended to assist reviewers and reduce the time to merge your change.

See https://gitlab.com/gitlab-org/quality/triage-ops/-/tree/master/doc/scheduled#testing-policies-with-a-dry-run on how to perform dry-runs for new policies.

See https://gitlab.com/gitlab-org/quality/triage-ops/-/blob/master/doc/reactive/best_practices.md#use-the-sandbox-to-test-new-processors on how to make sure a new processor can be tested.

Action items

• If adding environment variables for reactive processors, update config/triage-web.yaml and .gitlab/ci/triage-web.yml
• (If applicable) Add documentation to the handbook pages for Triage Operations =>
• (If applicable) Identify the affected groups and how to communicate to them:
  • /cc @person_or_group =>
  • Relevant Slack channels =>
  • Engineering week-in-review