test: add test case with indented trait header comments

Author: ytmimi

tests/source/trait.rs

@@ -124,6 +124,14 @@ auto /* post auto comment */ trait PubConstUnsafeAutoExampleWithComments {}
 
 pub /* first */ trait /* second */ MyTrait /* third */ {}
 
+pub
+   /* a */
+  trait
+    /* b */
+      Foo
+        /* c */
+{}
+
 // #3006
 trait Foo<'a> {
     type Bar<  'a  >;

tests/target/trait.rs

@@ -163,6 +163,14 @@ auto /* post auto comment */ trait PubConstUnsafeAutoExampleWithComments
 
 pub /* first */ trait /* second */ MyTrait /* third */ {}
 
+pub
+   /* a */
+  trait
+/* b */
+Foo /* c */
+{
+}
+
 // #3006
 trait Foo<'a> {
     type Bar<'a>;