Thank you MoonlightSentinel!

[Walter Bright]

I just noticed this PR by @MoonlightSentinel:

https://github.com/dlang/dmd/pull/10815

When you've got a test failure in fail_compilation, normally you'd get a dump of 
"expected output" and "actual output". Since this can be very long, it would 
make my eyes bleed trying to figure out what was actually different. I'd wind up 
cut/pasting the dump into separate files and then running a visual diff on that. 
This consumed a fair amount of time.

What MoonlightSentinal has done is add a "diff" section to the "expected" and 
"actual" sections. It's already saved me a big chunk of time.

Thank you!