Send a comment warning if the Bot doesn't have merge access

[wilzbach]

From #126 (comment)

For vibe-d/vibe-core#22, it turned out to be a simple authorization problem, so this just improves our test suite.

Btw maybe we should let the bot make a warning comment if it doesn't have access to a repo?
(OTOH this shouldn't happen that often)

[MartinNowak]

We have very few repos, don't sweat the small stuff.
But if there is interest, we should make the bot a github app to combine hooking and permissions.