Function test_host 

pub fn test_host<P>(content: impl Fn() -> P + 'static) -> impl Piece
where
    P: Piece,

The root of an app’s test build: the app’s content, or the driven case’s page alone. The root of an app’s test build: content normally, and while day test drives a GUI case, that case’s page alone, built fresh for the case and disposed when the next one starts. One page mounted at a time is what keeps one case’s ids, signals and resident native views out of the next one’s way.

pub fn root() -> impl Piece {
    day::test_host(|| my_app_root())
}

A panic while the case’s page is built is caught here and reported as the case’s failure; the host shows the message in its place.