Move jest unit tests to production folders (test/public-js to public/js)

• test helpers now live in ./__test__ so they are not mixed up with production and unit test files
• eslint config changed for the public/js folder

I was toying with an idea to use Language server to automate this refactoring so we could potentially revert it in the near future if this file order doesn't suit us. After a bit of exploring I decided that the manual option is going to be faster and even necessary because even when I moved files using VSCode, I still had to manually fix some references.

Testing strategy

• validate that all the tests are still running after this refactoring

  # the jest output both before and after
  Test Suites: 30 passed, 30 total
  Tests:       3 skipped, 264 passed, 267 total
  Snapshots:   66 passed, 66 total
  Time:        4.21s

• inspect the webpack bundle to validate jest doesn't get included in the production bundle
  • WEBPACK_REPORT=1 npm start
  • open ./webpack-report/index.html