Ensure valid class name in locustfile when filename contains special characters

[ivanpuchalka]

Not as trivial as I thought it'd be. Taking the definition in https://docs.python.org/3.8/reference/lexical_analysis.html#identifiers, the new code replaces any invalid character with an underscore.

Only added a simple test to the same file as the rest, I can add more if needed.

Fixes #14

[cyberw]

Nice!