add before/after scenario support

[unional]

before() is really not needed.

The main here is after, so that:

const { teardown, after, run } = await scenario(...)

after(() => teardown(...))

await run(...)

Alternative is make teardown delay before done, so that

const { teardown, setup, run, done } = await scenario(...)

teardown(...)

await setup(...)
await run(...)

await done() // done invoke registered teardown one by one