Gate tests with the right edition This PR guarantees that `./x test --test-args="--edition XXXX" ui` runs correctly with the 2015, 2018 and 2021 editions. I don't expect this PR to hold up over time but it helps to submit further updates to the `//@ edition` directives of tests where we can use the new range syntax to have a more robust testing across different editions r? `@fmease` --- try-job: aarch64-gnu try-job: aarch64-apple try-job: x86_64-msvc-1 try-job: i686-msvc-1 try-job: x86_64-mingw-1 try-job: test-various try-job: armhf-gnu