Clean up all DOM nodes when tests end

As the test suite grows it's bad for debugging to leak tons of elements
onto document.body. In order to concisely append/remove on the tests'
"end" event, we need to make sure the root tagName doesn't change. If it
does, the `tree` returned from `app.start` will no longer be the root
node and the removal fn will throw.

tests/browser/basic.js

@@ -1,4 +1,5 @@
 const test = require('tape')
+const append = require('append-child')
 const choo = require('../../')
 const view = require('../../html')
 
@@ -55,5 +56,5 @@ test('state is immutable', function (t) {
   ])
 
   const tree = app.start()
-  document.body.appendChild(tree)
+  t.on('end', append(tree))
 })