Enforce best practices for testing

[lindapaiste]

I'm not the biggest expert when it comes to testing components, so I looked up some guidelines and found that there's a lot of things which can be improved in existing unit tests.

See:

• Common mistakes with React Testing Library by RTL author Kent C Dodds

Changes:

• Installed eslint plugins for react testing library and jest-dom which will let us know when we're doing something wrong.
• Removed unnecessary act(() => {}) around render, fireEvent, etc. Which was actually every act in the code!
• Use specific assertions like toHaveValue(), toBeDisabled(), toBeChecked(), etc.
• Replace .mock.calls[0][0]).toBe() with toHaveBeenCalledWith().
• Use getBy* when accessing elements that should be present, and queryBy* only with .not.toBeInTheDocument();
• Fixed the buggy expectFileNameToBe checker which would always pass, as it had no expect condition and used a queryBy (which allows non-existance). It now uses an await find.

Not Fixed:

• I fixed some example code snippets in contributor_docs/testing.md, but this entire document could use a review.
• I put an // eslint-disable-next-line testing-library/no-render-in-setup in the client/index.integration.test.jsx file rather than fixing it.
• I turned off the rule testing-library/render-result-naming-convention because it doesn't like the way that we write const props = renderComponent(); in the Toolbar and FileNode tests.
• I have not switching to using the @testing-library/user-event library, as it caused errors.

I have verified that this pull request:

• has no linting errors (npm run lint)
• has no test errors (npm run test)
• is from a uniquely-named feature branch and is up to date with the develop branch.
• is descriptively named and links to an issue number, i.e. Fixes #123