[frontend] reset config changes after each test method
    
    We use following statements in our tests:
    
        app.config["foo"] = "bar"
    
    to *temporarily* change and test application behavior.
    The problem is, that we don't reset the config changes to their
    original values. As a consequence, test methods are not independent
    and it matters in what order they are executed, whether some
    tests were skipped and issues like #637 appear.
    
    I am chaning this and fixing the tests that rely on this bug.