Set up jest testing using React Testing Library

[AlanSl]

This builds on and supersedes https://github.com/nearform/libp2p-introspection-ui/pull/28

After getting everything working in that PR, I started building on it, but found the approach didn't scale for tests on more complex components where we need browser events that trigger hooks and change state. The thing we most need to test is that changes to state within context hooks containing data cause the expected updates in the expected places.

I've tried some things and found that react-testing-library works well for this (it's also one of the officially recommended options, and seems to support hooks much more cleanly than Enzyme).

So this PR takes the things that worked well from https://github.com/nearform/libp2p-introspection-ui/pull/28 -

• Snapshot testing using jest-styled-components
• Utils tests
• CI integration

...and adds React Testing Library (RTL), setting it up to encourage the kinds of tests we most need:

• Adding a custom testing utils package, @libp2p-observer/testing, which provides all tests with:
  • Three renderers that wrap in contexts: a light one with just the theme for very simple components, a semi-skimmed one with contexts exposing sample data and a selected timepoint, and a full-fat one including the full shell for testing integrations
  • Custom queries. RTL has a very sensible approach to encouraging tests that mimic user behaviour rather than testing brittle implementation details. This adds and uses a custom query analogous to a user scanning the page to see what is highlighted (avoiding tying our tests to brittle incidental details of how we implement highlighting)
  • A synchronous sample data loader for tests
• Updating existing tests from that PR; in particular making tests less tied to implementation details: "Does the function fire when the component is clicked" not "Does this particular HTML tag carry the onclick attribute"
• Updating other tests to use the new shared sample data loader
• Adding some more complex proof-of-concept tests to the ConnectionsTable component, testing that mouseovers on rows in the table highlight paths in the shell timeline and visa versa (also demonstrating that this approach is good for testing app-wide context updates in a user-driven way).

More SDK tests will be added as I pull this into the other PRs, it didn't make sense to add tests that would already be made obsolete by already-open PRs and WIP.

[AlanSl]

That should be all the comments addressed.

With the last couple of commits, code coverage is >90% of lines for all packages except Catalogue, which is fundamentally re-written in open PRs so it doesn't make sense to spend time covering obsolete code. Will get coverage >95% everywhere once the backlog is cleared and everything is up to date.