Allow some lints in .fixed files when compiling with Compiletest

[xFrednet]

See also Manishearth/compiletest-rs#243

Compiletest currently automatically allows all unused lints in the CompileFail and Ui modes. However, this doesn't extend to .fixed files, which are used inside Clippy. For this reason, we still have a bunch of #[allow()] attributes in Clippy's tests. It would be nice to have an option to allow lints inside .fixed compilation tests.

In a related discussion (See rust-lang/rust-clippy#7611), we agreed that it would also be cool, if we could be more specific than just allowing all lint's inside the unused group. I would suggest adding a new configuration with an enum like this:

pub enum LintConfiguration {
    /// No lints are automatically allowed
    None,
    /// All lints inside the `unused` lint group are allowed
    UnusedLints,
    /// List of lints that should be allowed. The names should be
    /// provided in the console format using dashes
    SpecificLints(&[&str])
}

Clippy uses compiletest-rs which is the downstream crate currently managed by @Manishearth. It would be great if this implementation could be done here and then transferred into compiletest-rs to keep them kind of in sync.

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I'm currently quite busy with RL stuff, but would be happy to work on this in about a month, if it would be accepted in rustc's compiletest version. Any feedback regarding the idea is welcome!

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cc: @rust-lang/devtools @Manishearth

@rustbot label +A-testsuite +C-enhancement +T-dev-tools