Create test suite for new EventRegistration Features

[kevinmirc]

Reference this PR and create test suites for the new features:

• EventRegistrationController#create
• EventRegistration#valid?
  • validates :ticket_class, presence: :true
  • validates :payment_intent_id
• EventRegistration#create_payment_intent
• EventRegistration#payment_intent
• EventRegistration#payment_intent=
• EventRegistration#only_requires_payment?