full coverage of ESLProtocol and some test utils

[Mazuh]

it's a challenge to implement new features or add fixes if we can't trust not having breaking changes, so while studying the lib I tried to help by adding full unit coverage of ESLProtocoL class.

before:

now:

I made some other changes too:

• ~wrapped _run property to make it more testable~
• added some blank lines for readability
• a recipe for test-coverage on makefile and test_requirements
• "polyfill" for mocks retrocompatible with python2

hope it helps. :)

[Mazuh]

also applied the changes orally requested by @italorossi