Optimize `dmr_rf` and `dmr_client` test features for faster tests

[sobolevn]

We provide our own fixtures that are subclasses of Django's builtin one.

Django has this code inside: https://github.com/django/django/blob/7dc826b9758d634623a6f5ca05d0ca2048a0ce48/django/test/client.py#L450-L458

    def _encode_json(self, data, content_type):
        """
        Return encoded JSON if data is a dict, list, or tuple and content_type
        is application/json.
        """
        should_encode = JSON_CONTENT_TYPE_RE.match(content_type) and isinstance(
            data, (dict, list, tuple)
        )
        return json.dumps(data, cls=self.json_encoder) if should_encode else data

We need to optimize this method, because json.dumps is slow, but msgspec is fast.
It won't be noticable on 1 tests, but will be on 2k tests (which is the case for our own test base).

See how we conditionally dump our schema here: https://github.com/wemake-services/django-modern-rest/blob/master/dmr/openapi/dump.py

Maybe we should reuse the same for tests?

[Nalsher]

Hi, can i work on this?

[sobolevn]

@Nalsher thanks a lot 🤝