Option to `#[allow]` additional lints

[GoldsteinE]

Currently trybuild automatically allows dead_code. I’d like to be able to allow other lints.

Use case: I have a macro that generates output with some #[cfg]s. These #[cfg]s mention Cargo features that are expected to be present in the caller crate. When used in trybuild-generated crate, they produce an unexpected_cfgs lint, which clutters .stderr files. I would like to disable unexpected_cfgs for all my trybuild test cases without manually writing #![allow] in every one. The syntax could look like this:

let t = trybuild::TestCases::new()
    .with_allowed_lint("unexpected_cfgs");
t.pass("path/to/file.rs");
t.compile_fail("another/file.rs");