Fix React warnings in useDocuments tests

[HerbCaudill]

When running tests we get several copies of this warning:

Warning: An update to TestComponent inside a test was not wrapped in act(...).

When testing, code that causes React state updates should be wrapped into act(...):

act(() => {
  /* fire events that update state */
});
/* assert on the output */

This ensures that you're testing the behavior the user would see in the browser. Learn more at https://reactjs.org/link/wrap-tests-with-act
    at TestComponent (automerge-repo/node_modules/.pnpm/@testing-library+react@14.1.2_react-dom@18.2.0_react@18.2.0/node_modules/@testing-library/react/dist/pure.js:278:5)
    at automerge-repo/packages/automerge-repo-react-hooks/test/useDocuments.test.tsx:90:37

This PR follows the warning's advice and wraps the stuff that causes state updates in act(...).

[pvh]

I mean, I don't see why but sure.