We now include the prelude in so many places, this simplifies how we can
present how `cargo-test-support` works.
Yes, this included some `use` clean ups but its already painful enough
walking through every test file, I didn't want to do it twice.
While this is noisy and hides other deprecations, I figured deprecations would
make it easier for people to discover what tasks remain and allow us to
divide and conquer this work rather than doing a heroic PR.
In theory, this will be short lived and we'll go back to seeing
deprecations in our tests.
`rustc` will start marking the suggestions for prefacing unused bindings
with underscores as "maybe incorrect", which makes them no longer auto
applicable by `rustfix`.
Change done at https://github.com/rust-lang/rust/pull/120470.
