Gather coverage from non-test runs

[kvark]

WebRender needs to test the coverage from (cd wrench && cargo run -- reftest) run under headless environment. We need a way to include this data. Just putting those tests into independent tests is not a good option, since recreating the context and reloading the shaders takes time.

[roblabla]

Wouldn't a (rather ugly) option be to have a test that runs the whole of reftest ? Like, putting all of https:/servo/webrender/blob/1fbc96b5295e03760e3863f78fd8b31e51e9284f/wrench/src/main.rs#L347 in a do_reftest function, and then doing

#[test]
fn pass_reftest() {
    do_reftest()
}

Otherwise, running kcov directly is also an option (kcov --verify --include=$PWD/src target/debug/wrench reftest)

My primary use-case for cargo-travis is that test filenames are unpredictable, and I wanted a solid way to get to them. When running a binary that has a static file name, I don't think it's terribly useful vs just running kcov.