Turn on jest env in test files only

Author: wojtekmaj

.eslintrc.json

@@ -1,11 +1,21 @@
 {
   "env": {
     "browser": true,
-    "es6": true,
-    "jest": true
+    "es6": true
   },
   "parserOptions": {
     "ecmaVersion": 2020
   },
-  "extends": "airbnb-base"
+  "extends": "airbnb-base",
+  "overrides": [
+    {
+      "files": [
+        "**/*.spec.js",
+        "**/__tests__/*"
+      ],
+      "env": {
+        "jest": true
+      }
+    }
+  ]
 }