Remove SlotMixin Test with wrong assumption about polyfill being loaded

[aghArdeshir]

Expected behavior

There is a test with title: does not scope elements when polyfill not loaded in context of SlotMixin - Scoped Registries, and there is a polyfill being loaded in the web-test-runner.config.mjs:

    <script src="/node_modules/@webcomponents/scoped-custom-element-registry/scoped-custom-element-registry.min.js"></script>

So I believe this test is having wrong assumption.

Actual Behavior

I created this draft PR as a proof: #2357

This test fails when polyfill is not loaded, but passes as soon as you add back the polyfill in the web-test-runner.config file.

Additional context

I really don't understand the context of these scoped elements really that much, so these changes are kinda done blindly, based on the obvious facts I observed (title of the test for instance)

I faced this blocker when trying to run every test with both polyfill loaded and without polyfill loaded for this PR: #2353

What I want to ideally achieve is to be able to get rid of this test. Remove it completely.

[tlouisse]

closed by #2407