Define ACCOUNT_SIGNUP_FORM_CLASS in test settings

Tests will fail when `ACCOUNT_SIGNUP_FORM_CLASS` is defined due to djstripe importing allauth modules before `StripeSubscriptionSignupForm` has been defined.

See #57

Also see pertinent code in allauth here:

https://github.com/pennersr/django-allauth/blob/master/allauth/account/forms.py#L146-L172