How to organise test files

Issue #327

In terms of tests, we usually have files for unit test, UI test, integeration test and mock.

Out of sight, out of mind.

Unit tests are for checking specific functions and classes, it’s more convenient to browse them side by side with source file. For example in Javascript, Kotlin and Swift

index.js
index.test.js
index.mock.js

LocationManager.kt
LocationManager.mock.kt
LocationManager.test.kt

BasketHandler.swift
BasketHandler.mock.swift
BasketHandler.test.swift

Integration tests check features or sub features, and may cover many source files, it’s better to place them in feature folders

- Features
  - Cart
    - Sources
    - Tests
      - Cart.test.swift
      - Validator.test.swift
  - Profile
    - Sources
    - Tests
      - Updater.test.swift
      - AvatarUploader.test.swift