Interceptor anchors for testing

[sparshev]

During testing of the shared libraries it's really useful to be able to intercept the calls.

Reason:

All the current implementations of the Jenkins Shared Libraries unit tests are quite useless - there are a number of issues impossible to test outside of actual workflow-cps-plugin. We have the next issue about that: JENKINS-33925.

[sparshev]

Added a sample realization #107 for Invoker.methodCall. That will be great, if someone can check - how this will work for the groovy-cps project and if this is the right way - to discuss it further.

[dwnusbaum]

Closing, follow JENKINS-33925 instead.