Test CI/CD deployments via requests to frontend

[Shabirmean]

Description

• Add ~unit~ integration tests to check if the frontend operates as expected!

[bourgeoisor]

I think we should still do this -- reseting SLO.

[NimJay]

Update since this is near out-of-SLO:

• I closed a similar bug https://github.com/GoogleCloudPlatform/point-of-sale/issues/92 because adding granular unit tests is not something our team can afford to do.
• However, judging from the description of this GitHub issue (#94), it sounds like this issue is about testing the frontend after it has been fully deployed — not unit testing the frontend with all its dependencies either mocked or disconnected.
• That would be a useful test, but "unit test" might not be the appropriate term.
• I have updated the name of this GitHub issue ("Add unit tests for the frontend" → "Test CI/CD deployments via requests to frontend"), and description.
• @Shabirmean correct me if I'm wrong, but I think adding such tests would involve:
  • adding a step inside some of the Cloud Build configs (e.g., pos-pr-open-to-main.yaml) that either makes raw curl HTTP requests or uses a tool like Cypress — similar to Bank of Anthos — to test various user journeys (e.g., add a BigBurger to cart, check out, and read the bill/receipt).

[Shabirmean]

@NimJay - Thank you once again for triaging these issues. Yes, I think it makes sense to focus on e2e like tests to ensure the App experience works as expected. Also, I agree with the description you have provided in terms of what such a test would look like.