Set FRESHMAKER_TESTING_ENV in testing env
    
    If run tests with 'python -m pytest', sys.argv[0] is:
    
        '${python_sitepackages_dir}/pytest/__main__.py'
    
    this breaks the check of testing environment (in 'init_config'),
    so we introduce a new env var "FRESHMAKER_TESTING_ENV" and set it
    in tox testenv.