UseSplitModeForUniqueDirectory configurability per test

[JanKrivanek]

Situation

UseSplitModeForUniqueDirectory is currently only a global option - it's not possible to be set per test (via VerifySettings instance)

Problem

The directory snapshot mode is now decision per whole module.
This can be especially problematic in situations where Verify is being used as part of wrapping tooling that wants to set the split mode for usages in it's own scope. Such tooling is then automatically incompatible with standard usage of Verify (due to possible attempt to set global setting after InnerVerifier has already run for unrelated test).

Proposed solution

Add members to VerifySettings to set the nullable flag with the same meaning. The current usage of VerifierSettings.UseUniqueDirectorySplitMode would prefer the local flag if not null.

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

[JanKrivanek]

Perfetto! Thank you Simon! 🙏