Update test suite

[rstacruz]

Just some updates to the test suite!

Jest update

• Update to latest Jest.
• Don't use puppeteer - instead, run the tests directly in Jest!
• Still works in the browser via Mocha

CI

• It should now run on GitHub actions, not Travis CI.
• It's now more reliable by running tests in Jest, rather than going through Puppeteer :)
• It will also check TypeScript types.

Removing features

• No linting -- Remove eslint and tslint for now.
• No gh-pages -- there was a script that publishes the test suite to the gh-pages branch. I don't think that's useful to anyone, so I removed it.