Improve _getrules to get all subclasses

[AmarOk1412]

Imagine we have a lot of TagRule objects with a lot of common code. It could be interesting to create a first rule with the common code and then inherit from it to make the other rules. Actually, this is not possible when using a single inheritance because of _getrules:

    def _get_rules(self, cls):
        """Returns a list of rules of a given class
        
        Rules are treated as singletons - we only instantiate each
        rule once. 
        """

        result = []
        for rule_class in cls.__subclasses__():
            rule_name = rule_class.__name__.lower()
            if rule_name not in self._rules:
                rule = rule_class(self)
                self._rules[rule_name] = rule
            result.append(self._rules[rule_name])
return result

for rule_class in cls.__subclasses__() only iterate over subclasses. So we have to do something like:

class TagTest(TestRule):
    def __init__(self, controller, severity=None):
        super().__init__(controller, severity)

    def configure(self, test_tags):
        self.test_tags = test_tags.split(',')

    def apply(self, testcase):
        continue

class TagScope(TagTest, TestRule): # Ugly hack for cls.subclasses
    def apply(self, testcase):
        continue

Improvement idea: use isinstance() instead of iterating over direct subclasses.

class Foo:
    foo = 1

class Bar(Foo):
    bar = 1

class Foobar(Bar):
    foobar = 1

isinstance(Foobar(), Foo) # True