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.
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