[8.x] Show a pretty diff for assertExactJson()

[jessarcher]

When using assertExactJson() with a large payload, it can be incredibly difficult to compare the "expected" and "actual" values to see what caused the failure.

This PR pretty-prints the JSON before asserting, which allows PHPUnit to show the specific lines that didn't match, with a few lines of context.

Before

After

I couldn't find a way to test this new behaviour which didn't just seem to be testing PHPUnit itself.

[timacdonald]

Can you please just push this one straight to main and by-pass the PR process, thanks.

-- A developer very, very keen on this change.

[michaeldyrynda]

I feel like this is something that could be part of Collision. What do you think @nunomaduro?

[dillingham]

A (slightly) related papercut: when expected is empty and actual isn't (or vice versa), could just say that.. in most cases don't need the payload. In every other case, the pretty print is awesome.. thank you for PRing it.