Add component hydration in .md E2E tests, refactor

[hippotastic]

Changes

• Our E2E tests for preact, react, solid and svelte were pretty similar, but had some subtle and unexpected differences (missing client:only tests, incomplete HMR tests). I therefore refactored these tests to use a common test factory, thereby bringing them to the same level of tested features, and also avoiding future differences.
• We did not have any islands hydration tests for components inside Markdown files. I fixed this and added Markdown tests for all of the frameworks mentioned above.

Testing

• I successfully ran the new E2E tests.

Docs

• Not a visible change.

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🦋 Changeset detected

Latest commit: 4756fe3

The changes in this PR will be included in the next version bump.

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[hippotastic]

@natemoo-re PTAL :) More test coverage as discussed.

[tony-sull]

Excellent cleanup, very cool! LGTM 🚀