Ensure visualizer operator override

[glopesdev]

The implementation in #1751 introduced a regression whereby Visualizer operators found themselves overruled by the new propagation semantics for Sink. This happened because Visualizer inherits its internal Process method from the Sink base class, and therefore the following check succeeded for both Sink and Visualizer since DeclaringType would be Sink in both cases:

bonsai/Bonsai.Core/Expressions/InspectBuilder.cs

Line 215 in 130de6f

methodCall.Method.DeclaringType == typeof(Reactive.Sink)) &&

This PR introduces a new internal virtual method to allow the Process method to be redeclared in derived classes. This allows Visualizer to "override" the method with a new static function which just calls the same base class implementation but which now will cause a change in the DeclaringType of the method. The above check will now fail for Visualizer types, which restores the behavior of the operator.

Due to the subtle nature of visualizer propagation semantics and the likelihood of further internal implementation reviews, we also added regression tests for the issue, together with a refactoring of the testing framework towards a more flexible combinator-based API for building workflow graphs (this could almost go into its own PR as it can in principle be used for everything, but I prefer to leave it close to its original motivation and we can see where it goes from there).

Fixes #1860

[PathogenDavid]

Implementation looks good, new tests catch intentionally regressing either issue as appropriate. Had some comments on the tests and new test infrastructure.

I didn't verify in-editor because lazy I assume Bruno will double-check it solves things on his end 😁

[bruno-f-cruz]

Working as intended on my end! Thanks

[PathogenDavid]