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).

docs/tests.md

@@ -1,16 +1,20 @@
-# Unit tests
+# Tests
+
+- 💡 You can use path/module alias `@/tests` in import statements.
+
+## Unit tests
 
 - Unit tests are defined in [`./tests`](./../tests)
 - They follow same folder structure as [`./src`](./../src)
 
-## Naming
+### Naming
 
 - Each test suite first describe the system under test
   - E.g. tests for class `Application` is categorized under `Application`
 - Tests for specific methods are categorized under method name (if applicable)
   - E.g. test for `run()` is categorized under `run`
 
-## Act, arrange, assert
+### Act, arrange, assert
 
 - Tests use act, arrange and assert (AAA) pattern when applicable
 - **Arrange**
@@ -23,7 +27,7 @@
   - Should elicit some sort of response
   - Starts with comment line `// assert`
 
-## Stubs
+### Stubs
 
 - Stubs are defined in [`./tests/stubs`](./../tests/unit/stubs)
 - They implement dummy behavior to be functional