Allow nesting of middleware.test functions

[chrisdickinson]

This would allow for the following:

tap.test('Frobnicators', _(async assert => {
  var frob
  tap.beforeEach(() => {
    frob = Frob.objects.create({...}) // recreated per-test in this test handler
  })

  tap.test('should frob evenly', _(async assert => {
    Frob.objects.filter({id: frob.id}).update({...}) // changes discarded before next test
  })
}))

Specifically we should track if we're in a test so we can use CHECKPOINT / ROLLBACK TO CHECKPOINT to persist some changes in the db. Redis... we'll continue to drop entirely between tests.