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 prof: #2357
This PR:

• Removes every other test in the SlotMixin test file
• Removes the polyfill from web-test-runner.config
• Filters only the tests in the core folder so we see the proof faster

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.