NestJS APITests

Setup: #32 (comment 85479)

Postman-kokoelma: https://www.getpostman.com/collections/d2c5efeae0a283afc8a9

• tracking https://www.getpostman.com/collections/ba8cd9b402851a0ba44e

• yksittäiset kyselyt https://www.getpostman.com/collections/30ef074827d510c197cd

Frontendille tapahtuva purku: ehasa-frontend#45