In writing the contrib documentation for functional vs ui tests, I
realized that as we work to make snapbox work for the functional tests,
we'll need distinct `Assert` objects since we'll want to elide a lot
more content in functional tests. I'm making room for this by
qualifying the existing asserts as being for "ui".
This only adds information about snapshot testing using `snapbox` and
keeps the functional testing documentation focused on the existing
facilities. We can updated this as our use of `snapbox` matures.
In writing this, I did notice that we define
`cargo_test_support::compare::assert` but only use it for filesystem
asserts and not binary asserts. We should probably add our own function
that wraps `snapbox::cmd::Command::cargo()` and passes in
`cargo_test_support::compare::assert`. I've left that out of this PR to
keep things focused.