RuntimeError: unable to define an event with event_kind that overlaps with an existing type

[exceptorr]

Originally inspired by canonical/operator#307

tl;dr Harness and non-Harness tests are interfering with each other, so we should not access charm object directly, like we did before.

We need to refactor an existing unit tests (e.g remove direct charm object references) to fix this:

======================================================================== FAILURES ========================================================================
_____________________________________________ OnConfigChangedHandlerTest.test__it_blocks_until_pod_is_ready ______________________________________________

self = <charm_test.OnConfigChangedHandlerTest testMethod=test__it_blocks_until_pod_is_ready>
mock_pod_spec = <function set_juju_pod_spec at 0x7fb334d35f80>, mock_juju_pod_spec = <function build_juju_pod_spec at 0x7fb334d4c710>
mock_time = <NonCallableMagicMock name='time' spec_set='module' id='140407662251216'>
mock_k8s_mod = <NonCallableMagicMock name='k8s' spec_set='module' id='140407662251984'>
mock_build_juju_unit_status_func = <function build_juju_unit_status at 0x7fb334d5e050>

    @patch('charm.build_juju_unit_status', spec_set=True, autospec=True)
    @patch('charm.k8s', spec_set=True, autospec=True)
    @patch('charm.time', spec_set=True, autospec=True)
    @patch('charm.build_juju_pod_spec', spec_set=True, autospec=True)
    @patch('charm.set_juju_pod_spec', spec_set=True, autospec=True)
    def test__it_blocks_until_pod_is_ready(
            self,
            mock_pod_spec,
            mock_juju_pod_spec,
            mock_time,
            mock_k8s_mod,
            mock_build_juju_unit_status_func):
        # Setup
        mock_fw_adapter_cls = \
            create_autospec(framework.FrameworkAdapter, spec_set=True)
        mock_fw_adapter = mock_fw_adapter_cls.return_value

        mock_juju_unit_states = [
            MaintenanceStatus(str(uuid4())),
            MaintenanceStatus(str(uuid4())),
            ActiveStatus(str(uuid4())),
        ]
        mock_build_juju_unit_status_func.side_effect = mock_juju_unit_states

        mock_event_cls = create_autospec(EventBase, spec_set=True)
        mock_event = mock_event_cls.return_value

        harness = Harness(charm.Charm)
>       harness.begin()

test/charm_test.py:93:
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
lib/ops/testing.py:121: in begin
    self._charm = TestCharm(self._framework, self._framework.meta.name)
src/charm.py:39: in __init__
    super().__init__(*args)
lib/ops/charm.py:353: in __init__
    self.on.define_event(relation_name + '_relation_created', RelationCreatedEvent)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

cls = <class 'ops.testing.Harness.begin.<locals>.TestEvents'>, event_kind = 'http_api_relation_created'
event_type = <class 'ops.charm.RelationCreatedEvent'>

    @classmethod
    def define_event(cls, event_kind, event_type):
        """Define an event on this type at runtime.

        cls: a type to define an event on.

        event_kind: an attribute name that will be used to access the
                    event. Must be a valid python identifier, not be a keyword
                    or an existing attribute.

        event_type: a type of the event to define.

        """
        prefix = 'unable to define an event with event_kind that '
        if not event_kind.isidentifier():
            raise RuntimeError(prefix + 'is not a valid python identifier: ' + event_kind)
        elif keyword.iskeyword(event_kind):
            raise RuntimeError(prefix + 'is a python keyword: ' + event_kind)
        try:
            getattr(cls, event_kind)
            raise RuntimeError(
>               prefix + 'overlaps with an existing type {} attribute: {}'.format(cls, event_kind))
E               RuntimeError: unable to define an event with event_kind that overlaps with an existing type <class 'ops.testing.Harness.begin.<locals>.TestEvents'> attribute: http_api_relation_created

lib/ops/framework.py:322: RuntimeError