Add test warning on Frontstore

[joyet-simon]

Reason for change

Linear task

Code changes

Add a param in plans formatted for checkout and if we are in test mode, add ma warning in the checkout.

How to test

As a reviewer, you are encouraged to test the PR locally.
Go to the Storefront in test mode and see the warning on each Alma payment method.

Checklist for authors and reviewers

• The title of the PR uses business wording, not technical jargon, for the changelog readers to understand it
• The PR implements the changes asked in the referenced task / issue
• The automated tests are compliant with the testing strategy
• The tests are relevant, and cover the corner/error cases, not only the happy path
• You understand the impact of this PR on existing code/features
• The changes include adequate logging and Datadog traces
• Documentation is updated (API, developer documentation, ADR, Notion...)

Non applicable

[sonarcloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud