Support "soft" feature-gating using a lint

Use it for feature-gating `#[bench]`

src/test/ui/feature-gates/bench.rs

@@ -0,0 +1,4 @@
+#[bench] //~ ERROR use of unstable library feature 'test'
+fn bench() {}
+
+fn main() {}

src/test/ui/feature-gates/bench.stderr

@@ -0,0 +1,10 @@
+error: use of unstable library feature 'test': `bench` is a part of custom test frameworks which are unstable
+  --> $DIR/bench.rs:1:3
+   |
+LL | #[bench]
+   |   ^^^^^
+   |
+   = note: `#[deny(soft_unstable)]` on by default
+
+error: aborting due to previous error
+