diff_test Unified vs Context Output

For the oses that use a real diff binary, can we add an option to select unified vs context diff output (`-c` vs `-u`)?

I don't mind putting in a PR, if this is something that would be accepted 😸

Start by locating diff_test and the code paths used on operating systems that invoke a real diff binary. Trace how the current diff mode is selected, then add coverage for both unified and context output and verify the option works on the supported operating systems.

Written by the indexing model from the issue text.