REFACTOR: Move test setup to a module (PR #10860)

This is long overdue. We had a lot of (not linted) code to initialize our test suite as part of the Ruby test_helper.js bundle.

This refactor moves that out to a setup-tests module, which imports all the modules properly, rather than using require.

It also removes the global server variable which some tests were using for pretender. Those tests are fixed, and in the case of widget tests, support for a pretend() was added, which mimics our acceptance tests.

One problematic test was removed, which overwrites /posts - this could break tons of other tests depending on order.