Fix error message printing wrong file names
Fix error message printing wrong file names

The error messsages from `self_test` does not display correct file names
(it just prints `src/lib.rs` for every file). This commit fixes it.

aidancully/rust,rust-lang/rustfmt,aidancully/rust,SingingTree/rustfmt,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,rust-lang/rustfmt,aidancully/rust,aidancully/rust,graydon/rust,SingingTree/rustfmt,graydon/rust