Remove useWysiwyg mock

[florianduros]

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)

Removed the mock for useWysiwyg. During the first implementation here, issues with the wasm in the jest env forced to mock the hook.

• The Response object is undefined in the jest env and make the wasm download failed.
• With the hook properly working, I refined some tests to make them more reliable.

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