slapos_subscription_request: Partially refactor and extend scenario tests

Keep source[_section] None on the New template tests

   This feature (with values set) is covered on Dual organisation settings.
   Unify code into mixin for Chinese scenarios
   Update Payzen for the new API
   Remove duplication and fix up assert
   Implement Test for Parallel subscriptions

      Test 2 organisations rather them a single one with parallel Subscriptions
      Move more code into the Mixin, for reuse it.