Hey!… Leave me alone! :) But seriously. I never understood how to test things. I can understand testing, say, addition function or something. You pass 2 parame

Hey!… Leave me alone! :) But seriously. I never understood how to test things. I can understand testing, say, addition function or something. You pass 2 parameters, you have expected results, if return value does not match expected results - test fails. OK. But how do you test complex business logic? Like, i don’t know, authentication? Say I have a list of rules user needs to fulfill to be logged in (subscription, account in good standing, username starts with emoji, etc.). How do I test that?