After each test, remove any stray nodes added to the document.

This was not necessary when we were running each test in its own
`<iframe>`, and it doesn't seem to affect any test behavior currently, but
it seems wise for the sake of test isolation and hygiene.
Rename removeSiblings to removeNextSiblings.
Merge pull request #213 from benjamn/remove-stray-nodes-after-each-test

After each test, remove any stray nodes added to the document