bors sometimes makes PRs haunted

[Noratrieb]

it happened here https://github.com/rust-lang/rust/pull/117494

• a different PR, https://github.com/rust-lang/rust/pull/121523, started being tested
• CI failed
• the tree was closed after a retry and then tried a few times
• meanwhile, our friend 117494 was being try built
• after the tree was reopened, somehow 117494 started being shown as currently testing
• the try build completed
• probably because the "Mark job as successful" job doesn't run in try, bors thought that it hasn't completed
• Several hours later, it was still marked as testing, blocking the queue
• I did bors retry, throwing it out and unblocking the queue
• I closed the PR anyways because I was afraid bors would just push to master after the try build
• I reopened the PR and tried again
• it was now also makes as testing, not trying!
• I closed the PR
• PR's haunted