Allow DiffPlex injection/configuration per test

[JanKrivanek]

Situation

Currently the initialization is only global, there is no option to configure/alter it per test

Problem

Analogous to VerifyTests/Verify#757.
It's rather edge scenario (Verify being used within wrapping tooling), but it's quite painful there and fix is nonbreaking and easy

Proposed solution

Add a call that accepts VerifySettings instance and configures it via VerifySettings.UseStringComparer

I'm more than happy to create PR for this if this request is acknowledged and agreed

[JanKrivanek]

Very appreciated! Thank you!