Add unit tests for remaining patterns

[fluxw42]

Added unit tests for:

• twin pattern
• tolerant-reader pattern
• thread-pool pattern (also fixed concurrency problem in id generation of Task)
• template-method pattern
• visitor pattern
• strategy pattern
• step-builder pattern
• state pattern
• specification pattern
• singleton pattern
• service-locator pattern (also fixed NPE when requested service is unknown)
• service-layer pattern
• servant pattern
• resource-acquisition-is-initialization pattern
• proxy pattern
• prototype pattern
• property pattern
• producer-consumer pattern
• private-class-data pattern
• poison-pull pattern
• generic observer pattern
• observer pattern
• object-pool pattern
• null-object pattern
• multiton pattern

With this merge you should be able to close issue #293.

[iluwatar]

@fluxw42 great work once again. I'm impressed by your mastery of JUnit and Mockito 👍

[fluxw42]

@iluwatar Thanks! It's not really 🚀-science 😃.