add module alias '@tests/'

Alias would remove unnecessary repetitions and less relative paths make changes easier when moving around files. This commit cleans also up some relative paths ('../../../') by using the alias and orders imports. It updates both path alias in tsconfig and module alias in Vue CLI's bundler (vuejs/vue-cli#2398).

tests/unit/application/Environment/BrowserOs/BrowserOsDetector.spec.ts

@@ -1,3 +1,4 @@
+import 'mocha';
 import { expect } from 'chai';
 import { OperatingSystem } from '@/domain/OperatingSystem';
 import { BrowserOsDetector } from '@/application/Environment/BrowserOs/BrowserOsDetector';