Improve coverage of unit tests to over 80%

[cmagnobarbosa]

Today of_core NApp has unit test coverage below 80% to follow EP021 and change the color of the test panel (https://kytos.io/tests) to green (coverage >= 80%) instead of orange (coverage 60~79) it is necessary to improve this coverage to a minimum of 80%.

Colors on the test panel:

• <60 red, <80 orange, >=80 green.