Refactor tests to resolve the pylint messages

[nathanaelbosch]

At the time of writing, tests triggers the following pylint messages:

• line-too-long
• duplicate-code
• missing-class-docstring
• unnecessary-pass
• unused-variable
• protected-access
• attribute-defined-outside-init
• no-self-use
• abstract-class-instantiated
• too-many-arguments
• too-many-instance-attributes
• too-many-locals
• unused-argument
• fixme
• missing-module-docstring
• missing-function-docstring

It might also be that we do not actually want to fix all of these for the tests, so if you have an opinion on this please share it by commenting on this issue.

Goal:
Our long-term goal is to enable all of these checks for all parts of the project. Therefore, we should continuously work on fixing these. Currently, pylint is set up to have different checks for different modules, such that we can work on these problems individually and progress more quickly :)

To work on this issue you can check for these messages individually by calling:

pylint tests --disable=all --enable=<message>

Then, after fixing all errors/warnings, you should remove the exception from the corresponding line in ./tox.ini, such that this message will be tracked in the future.

[pnkraemer]

I conjecture that refactoring the tests so that pylint is happy will be painful... I suggest to tackle this last ;)

[marvinpfoertner]

Superseded by #523.