test: implement use of existing subproject in tests

Konstantin Khottchenkovrequested to merge
slb/kk/subproject into master
  1. Implement use of existing subproject in tests. New subproject will be created if the name of subproject is not provided or subproject doesn't exist.
  2. Single response code expected in tests
  3. Minor changes in postman collection
Edited by Diego Molteni

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