Add test for bad assignments (to self/cls) about pep8-naming

[sigmavirus24]

This plugin is simply for names of functions, methods, and classes. It does not check the bodies for assignments and neither should it. Those are valid assignments in practice although they may introduce unexpected behaviour there is no reason for this plugin to squawk about them.

[skirpichev]

This plugin is simply for names of functions, methods, and classes.

VariablesInFunctionCheck checks also variable names in function body, just like this test.

[sigmavirus24]

I misspoke. Regardless the point stands that this is not a issue of naming but rather something better suggested for a linter not a style checker