tests: silence deprecation warning for fido2.features.webauthn_json_m…

…apping.enabled

future function would change currently expected data representation

```
  You are using deprecated functionality which will change in the next major version of
  ...
  This changes the keys and values used by the webauthn data classes when accessed using
  the Mapping (dict) interface (eg. user_entity["id"] and the from_dict() methods) to be
  JSON-friendly and align with the current draft of the next WebAuthn Level specification.
  For the most part, this means that binary values (bytes) are represented as URL-safe
  base64 encoded strings instead.
```

also concerns
#13

tests/example_server.py

@@ -44,6 +44,8 @@
 from flask import Flask, session, request, redirect, abort
 
 import os
+import fido2.features
+fido2.features.webauthn_json_mapping.enabled = False
 
 
 app = Flask(__name__, static_url_path="")