Add test for issue 3937
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.

rust-lang/rustfmt,SingingTree/rustfmt,rust-lang/rustfmt,SingingTree/rustfmt