Auto merge of #9698 - ehuss:enable-future-incompat, r=alexcrichton
Auto merge of #9698 - ehuss:enable-future-incompat, r=alexcrichton

Re-enable future-incompatible tests.

This re-enables the future-incompatible tests that were disabled in #9638.

rustc now has a `-Zfuture-incompat-test` flag we can use to test the future-incompatible reporting system. It causes rustc to treat every lint as future-incompatible, so we can just pick some arbitrary lint that should be stable over time.

rust-lang/cargo,nipunn1313/cargo,nipunn1313/cargo,eminence/cargo,eminence/cargo,nipunn1313/cargo,rust-lang/cargo,rustyhorde/cargo,rustyhorde/cargo,rustyhorde/cargo,eminence/cargo,rustyhorde/cargo,nipunn1313/cargo,eminence/cargo,rust-lang/cargo,rust-lang/cargo