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).

tsconfig.json

@@ -25,7 +25,10 @@
     "paths": {
       "@/*": [
         "src/*"
-      ]
+      ],
+      "@tests/*": [
+        "tests/*"
+      ],
     },
   },
   "include": [
@@ -38,4 +41,4 @@
   "exclude": [
     "node_modules"
   ]
-}
+}