add initial smoke tests and documentation

What does this MR do?

Adds initial smoke test functionality for quick sanity testing from the UI.

Are there points in the code the reviewer needs to double check?

There will be a sister MR to this created in the QA Repository regarding documentation on how to run these smoke tests.

We can run them by using:

bin/qa Test::Instance::Smoke http://localhost:3000

I ran the "smoke suite" a couple times: here are my findings.

Time taken
50.9s
50.08s
50.78

Why was this MR needed?

This MR was created per discussions in gitlab-qa#288 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conform by the code review guidelines
  • Has been reviewed by a Frontend maintainer
  • Has been reviewed by a Backend maintainer
• Conform by the merge request performance guides
• Conform by the style guides
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

• gitlab-qa#288 (closed)

Notes

We should of course expand this suite, perhaps adding more describe scenarios in our source base to what we believe should be constituted as a "smoke test."

/cc @gl-quality @rspeicher @grzesiek @stanhu