coverage: Add a test that uses #[bench]

tests/coverage/bench.cov-map

@@ -0,0 +1,16 @@
+Function name: bench::my_bench
+Raw bytes (9): 0x[01, 01, 00, 01, 01, 08, 01, 00, 27]
+Number of files: 1
+- file 0 => global file 1
+Number of expressions: 0
+Number of file 0 mappings: 1
+- Code(Counter(0)) at (prev + 8, 1) to (start + 0, 39)
+
+Function name: bench::my_bench::{closure#0}
+Raw bytes (9): 0x[01, 01, 00, 01, 01, 07, 01, 00, 09]
+Number of files: 1
+- file 0 => global file 1
+Number of expressions: 0
+Number of file 0 mappings: 1
+- Code(Counter(0)) at (prev + 7, 1) to (start + 0, 9)
+

tests/coverage/bench.coverage

@@ -0,0 +1,9 @@
+   LL|       |#![feature(test)]
+   LL|       |// edition: 2021
+   LL|       |// compile-flags: --test
+   LL|       |
+   LL|       |extern crate test;
+   LL|       |
+   LL|      1|#[bench]
+   LL|      1|fn my_bench(_b: &mut test::Bencher) {}
+

tests/coverage/bench.rs

@@ -0,0 +1,8 @@
+#![feature(test)]
+// edition: 2021
+// compile-flags: --test
+
+extern crate test;
+
+#[bench]
+fn my_bench(_b: &mut test::Bencher) {}