I think what's going on is that GitHub takes some non-zero amount of time to compute this value, and our test runs too fast sometimes and try to fetch a PR before its mergeability is computed
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
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.
