Test: don't unmount MatrixChat before setup is finished

[kerryarchibald]

Failure ex: https://github.com/matrix-org/matrix-react-sdk/actions/runs/5538102798/jobs/10107690836?pr=11249

1. Loading test mounts MatrixChat
2. MatrixChat starts doing a lot of setup
3. Loading test doesn't wait for setup to finish
4. test cleanup unmounts MatrixChat, which unsets MatrixClient on MatrixClientPeg
5. MatrixChat continues setting up and tries to use an unset client from MatrixClientPeg.get()

Adds more waiting to loading test to avoid unmounting before app startup settles.

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.

[andybalaam]

Thank you!