That's how we build using Jenkins - it's a nice feature, but it doesn't retrigger when the target branch changes. So, while it's useful so far as that it's better than simply building the branch, it doesn't ensure that "what you tested is what you get."
In the Rust project, the merging itself is automated and the merge bot (bors) runs the test after having effected the merge. The merging is attempted after the branch has passed review.
That doesn't preclude running e.g. travis to get the branch's own status before the review happens.
