build: Make coverage output actually usable

Coverage used to calculated over everything including
previous coverage calculations which is not exactly useful.

Now it just inspects the source files which seems a much more
sensible approach.

Author: eFrane

jest.config.js

@@ -8,8 +8,8 @@ module.exports = {
   coverageDirectory: 'coverage',
 
   collectCoverageFrom: [
-    "**/*.js",
-    "!**/node_modules/**"
+    "src/**/*.js",
+    "!**/node_modules/**",
   ],
 
   notify: true,