[ktest] simplification and refactoring

I finally made the changes that I had in mind for a while.

* Instead individually passing settings, `KTestOptions` object is passed between classes. This became a problem after we had more and more global settings.
* I removed string constants. Since they can't be used in annotations, most of them were left unused after moving to JCommander. For uniformity I removed the rest.
* Some various unused stuff removed.
* ~~Injected `KExceptionManager` is used instead of the one from `GlobalSettings`.~~

Just to make sure I'll run test suite on Windows manually.