Create tests for Nonce model and logic

[payton]

Write unit tests for...

1. nonce_is_valid function

   django-siwe-auth/siwe_auth/backend.py

   Lines 28 to 39 in 12c6d95

   	def _nonce_is_valid(nonce: str) -> bool:
   	"""
   	Check if given nonce exists and has not yet expired.
   	:param nonce: The nonce string to validate.
   	:return: True if valid else False.
   	"""
   	n = Nonce.objects.get(value=nonce)
   	is_valid = False
   	if n is not None and n.expiration > datetime.datetime.now(tz=pytz.UTC):
   	is_valid = True
   	n.delete()
   	return is_valid

2. Nonce model itself

   django-siwe-auth/siwe_auth/models.py

   Lines 74 to 79 in 12c6d95

   	class Nonce(models.Model):
   	value = models.CharField(max_length=24, primary_key=True)
   	expiration = models.DateTimeField()
   	def __str__(self):
   	return self.value

3. Nonce scrubbing logic (with freezegun?)

   django-siwe-auth/siwe_auth/views.py

   Lines 53 to 68 in 12c6d95

   	@ratelimit(key='ip', rate='5/m')
   	@require_http_methods(["GET"])
   	def nonce(request):
   	now = datetime.now(tz=pytz.UTC)
   	_scrub_nonce()
   	n = Nonce(value=secrets.token_hex(12), expiration=now + timedelta(hours=12))
   	n.save()
   	return JsonResponse({"nonce": n.value})
   	def _scrub_nonce():
   	# Delete all expired nonce's
   	for n in Nonce.objects.filter(expiration__lte=datetime.now(tz=pytz.UTC)):
   	n.delete()