Separate out clean-up of Pull requests created during test

If the test assertion fails, we want the pull-request to be closed anyway.
You can't have more than one pull-request merging a given branch into
another, so leaving the PR hanging will cause subsequent test runs to fail
in setup.
Fix setting labels and assignee on PullRequests

Setting labels and assignee on Pull requests failed silently, because
the API endpoint being hit contained '/pulls/' rather than '/issues/'.

"Every pull request is an issue, but not every issue is a pull request.
For this reason, “shared” actions for both features, like manipulating
assignees, labels and milestones, are provided within the Issues API."

https://developer.github.com/v3/pulls/#labels-assignees-and-milestones
Merge pull request #123 from rtyley/use-issues-endpoints-for-pull-requests

Use issues endpoints for pull requests