Add tests for createEditContent which mirror tests for createMessageContent.

[clokep]

Adds some tests for createEditContent. They're essentially the tests for createMessageContent with modifications to make sense for edits. This PR has no behavior changes for real code.

I want to add additional tests here for #9983, but split out some basic tests first since those are unrelated (and unchanged).

Note that the last test kind of gives unexpected behavior, but I made it pass by adding the content which matrix-react-sdk generates, instead of the expected content a user would want. I don't see an open bug for this (maybe introduced in #3893 though?) and attempted to fix it quickly but did not have success.

Checklist

• Tests written for new code (and old code if feasible)
• Linter and other CI checks pass
• Sign-off given on the changes (see CONTRIBUTING.md)

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This change is marked as an internal change (Task), so will not be included in the changelog.

[florianduros]

Thanks for tests!

FYI, a rich text editor is also available on the labs (which should replace this composer in the futur)