[CI] js: test: Decouple screenshots from regular e2e tests

[ix5]

Checklist

• All new and existing tests are passing
• (If adding features:) I have added tests to cover my changes
• (If docs changes needed:) I have updated the documentation accordingly.
• I have added an entry to CHANGES.rst because this is a user-facing change or an important bugfix
• I have written proper commit message(s)
  

What changes does this Pull Request introduce?

Decouple the screenshot tests from integration test suite.

Why is this necessary?

Screenshot tests are a bit confusing for beginners.

[ix5]

Python tests are currently broken due to pallets-eco/flask-caching#352
If the flask-caching maintainers don't fix this issue in time, we'll need to add cachelib to our own deps or pin an older version of flask-caching.