Improve error message in make_tests
Improve error message in make_tests

We should use expect instead of unwrap.

This commit is based on https://github.com/laumann/compiletest-rs/pull/58. Thanks to @colin-kiegel.
graydon/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust