Switch from Karma/Jasmine to Jest

Author: fritz-c

.eslintrc.json

@@ -2,7 +2,7 @@
   "extends": ["airbnb", "prettier", "prettier/react"],
   "env": {
     "browser": true,
-    "jasmine": true
+    "jest": true
   },
   "rules": {
     "react/jsx-filename-extension": 0,

__mocks__/fileMock.js

@@ -0,0 +1 @@
+module.exports = 'test-file-stub';