[LIT] Provide an alternative diff UI

[glyh]

After some of my experience with LIT, I feel the error message provide while being informative, but not the most readable one.

Thus it would be nice if we design a new interface for lit, ideally would yield error message like https://github.com/dandavison/delta, or just provide a general interface and let end user decide how the unit test failure should be reported.

For example, here's git show on my system, I have a 2 paneled view to compare