assertion diff for "Expected" is barely readable

[littledivy]

Bg green on white text makes it very hard to read anything. It should have some contrast.

https://github.com/denoland/deno_std/blob/fa6c014902d834203914afa9a77d83c69c60c228/testing/_diff.ts#L383

[lucacasonato]

Agreed. Black on green seems much better.