Allow nesting of middleware.test functions

This would allow for the following:

```js
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.