Improvements to testing

[zivnevo]

Suggestions from @vikin91 :

• use assertion package to reduce the amount of code and unify the failure messages (e.g., https://github.com/stretchr/testify)
• convert the isolated test cases into table-driven tests, so that we can add new test cases with little effort
• make sure output files do not collide with each other so that tests can run in parallel
• Add tests to the analyzer package
• precisely define the coverage of the tests, so that we cover the small slices of the system and we do not need to adapt all the test cases when we change only a part of the processing path. In particular, I am thinking about something like:
  • what do we test - file reading, yaml splitting, yaml parsing?
  • shall we have tests only for connection extraction?
  • shall we have tests only for netpol senthesizing?