Add test for issue 3937

Closes 3937

It's unclear which change fixed the `format_code_in_doc_comments=true`
issue brought up in this issue, however I'm unable to reproduce the
error on the current master.

The added test cases should serve to prevent a regression.

Author: ytmimi

tests/target/issue_3937.rs

@@ -0,0 +1,13 @@
+// rustfmt-format_code_in_doc_comments:true
+
+struct Foo {
+    // a: i32,
+    //
+    // b: i32,
+}
+
+struct Foo {
+    a: i32,
+    //
+    // b: i32,
+}