rustdoc-gui/tester: print full filename on error
rustdoc-gui/tester: print full filename on error

This makes it easier to find the test that needs fixing.

aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust