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.