Add unit tests for Watcher and Master

[MateusDantas]

• [x] Create interfaces for ProcPreparable and ProcContainer. PR #3
• [ ] Create ProcPreparable and ProcContainer mock instances
• [ ] Implement unit tests for Watcher
• [ ] Implement unit tests for Master

Creating interfaces for ProcPreparable, ProcContainer and file_util is essential to make unit testing easier. After that we can easily create mock instances and apply it on master and watcher. The tests should mainly test the following:

• Starting processes serially
• Starting processes in parallel
• Killing processes serially
• Killing processes in parallel
• Killing keep-alive processes
• Killing non-keep-alive processes
• Trying to build non-compilable projects

The main idea is to test how APM behaves when facing possible race condition situations and how it handle errors.