feat(tests): add check for "owl-bot-staging"

[bshaffer]

It's possible for the owl bot processor to fail, which leaves owl-bot-staging dirs committed to the branch in the PR.

Although this is an obvious issue when reviewing the PRs, because of the high-volume of these Pull Requests that we receive (and the fact that we hope to completely automate them someday), it would be nice to have a check which fails when the owl-bot-staging directory exists, as this is NEVER something we want committed to the main branch.