I'd at some point drawn up a proposal for this, back when Rust started having problems with this. At the time I think we didn't really have the budget for such testing either.
bors' rollup support only lets one mark things for rollup, which show up differently in the queue. It's effectively a priority=-1. This is for things which probably won't break the build, and contributors make a batch rollup every now and then (bors has a button for this). Generally when I do rollups I go through these and ensure they're green on travis.
I've considered giving homu (the engine behind bors) better support for automatically doing this (at least automatically showing travis statuses) but I don't have much time.
bors' rollup support only lets one mark things for rollup, which show up differently in the queue. It's effectively a priority=-1. This is for things which probably won't break the build, and contributors make a batch rollup every now and then (bors has a button for this). Generally when I do rollups I go through these and ensure they're green on travis.
I've considered giving homu (the engine behind bors) better support for automatically doing this (at least automatically showing travis statuses) but I don't have much time.