Enhance NRules.Testing to work with rules that don't originate from Fluent DSL

[snikolayev]

This enhancement is coming from gitter: https://matrix.to/#/!wMbiwUicloLHQmHGXD:gitter.im/$9zOjjbrKTXeRE1WRnG6ojn2Rcoy9o513_J9gFWvAY_U?via=gitter.im&via=matrix.org

Right now, a test fixture assumes the rules are defined as Rule classes using fluent DSL. So, it cannot be used to test rules defined externally, where only IRuleDefinition is available.