Asynchronously declared nested tests w/ plan() w/o end()

Since `substack/tape` shares many of the same tests with `isaacs/node-tap` (I assume the tests here came from `node-tap`), I am adding a test from `node-tap` that I wrote to `tape`. 

Recently I added (to `node-tap`) support for asynchronously declaring nested tests to `node-tap` when you've given a plan() (the plan() lets it knows to wait for asynchronously created nested test harnesses). I added this same test to `tape` in this pull request. No further modifications were needed to `tape` since this test passed right out of the box. ... Plus it never hurts to have more tests. :)

Here is the PR on `node-tap` from where this test was pulled from: https://github.com/isaacs/node-tap/pull/103