Add eslint check for focused tests

[allisonking]

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Is your feature request related to a specific problem?

When developing, it is handy to use .only on tests. However, we don't want this checked in.

Describe the solution you'd like

Add an eslint plugin which will fail linting if there are focused tests.

Describe alternatives you've considered, if any

A description of any alternative solutions or features you've considered.

Additional context

Some possibilities: https://www.npmjs.com/package/eslint-plugin-no-only-tests

eslint-plugin-jest also has this, but it takes a bit of work to have it play nicely with Cypress: https://github.com/jest-community/eslint-plugin-jest. might be nice to have its other features though. Ideally something like this would go in a common place for all of the UI's to pull from