Add a test for coverage attr on trait function

Author: ehuss

tests/coverage/attr/trait-impl-inherit.cov-map

@@ -0,0 +1,9 @@
+Function name: <trait_impl_inherit::S as trait_impl_inherit::T>::f
+Raw bytes (9): 0x[01, 01, 00, 01, 01, 0f, 05, 02, 06]
+Number of files: 1
+- file 0 => global file 1
+Number of expressions: 0
+Number of file 0 mappings: 1
+- Code(Counter(0)) at (prev + 15, 5) to (start + 2, 6)
+Highest counter ID seen: c0
+

tests/coverage/attr/trait-impl-inherit.coverage

@@ -0,0 +1,24 @@
+   LL|       |// Checks that `#[coverage(..)]` in a trait method is not inherited in an
+   LL|       |// implementation.
+   LL|       |//@ reference: attributes.coverage.trait-impl-inherit
+   LL|       |
+   LL|       |trait T {
+   LL|       |    #[coverage(off)]
+   LL|       |    fn f(&self) {
+   LL|       |        println!("default");
+   LL|       |    }
+   LL|       |}
+   LL|       |
+   LL|       |struct S;
+   LL|       |
+   LL|       |impl T for S {
+   LL|      1|    fn f(&self) {
+   LL|      1|        println!("impl S");
+   LL|      1|    }
+   LL|       |}
+   LL|       |
+   LL|       |#[coverage(off)]
+   LL|       |fn main() {
+   LL|       |    S.f();
+   LL|       |}
+

tests/coverage/attr/trait-impl-inherit.rs

@@ -0,0 +1,23 @@
+// Checks that `#[coverage(..)]` in a trait method is not inherited in an
+// implementation.
+//@ reference: attributes.coverage.trait-impl-inherit
+
+trait T {
+    #[coverage(off)]
+    fn f(&self) {
+        println!("default");
+    }
+}
+
+struct S;
+
+impl T for S {
+    fn f(&self) {
+        println!("impl S");
+    }
+}
+
+#[coverage(off)]
+fn main() {
+    S.f();
+}