rustdoc: test ignoring rustc lints in CLI

[notriddle]

This works, but I couldn't find any tests for it.

[rustbot]

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[lolbinarycat]

the commit/PR title seems backwards, shouldn't it be "rustdoc: test denying rustc lints via CLI"?

in which case, we're not actually testing that, since the entire point of the modified test is that the lint doesn't pass.

if you actually wanted to make sure lint flags for rustc lints work properly, you'd have to find an allow lint for a check-pass test, or a deny lint for a check-fail test.

[notriddle]

Maybe I should title it: "rustdoc: test equivalent treatment of unused_imports on CLI and attr"? It's kind of long, but that's the real point of this test.