Rollup merge of #106888 - GuillaumeGomez:tidy-gui-test, r=notriddle

Add tidy check to ensure that rustdoc GUI tests start with a small description

The first commit comes from https://github.com/rust-lang/rust/pull/106865 to prevent CI to fail.

This PR adds a tidy check to enforce having a small description at the top of the GUI test. Although the format is made to be as easy as possible to read, it's not always obvious what a test is actually checking. I think enforcing this will make it easier for us to come back on these tests if needed.

r? `@notriddle`