reapply eslint to tests, still supporting jest

The full test suite failed because it tried to lint the tests, while the running linter did not do the same. Enabling linting gave errors due to jest having undef stuff, but overriding these files for jest env keeps the linting without these errors.

Author: Gramatus

.eslintrc.json

@@ -12,5 +12,12 @@
     },
     "rules": {
     },
-    "ignorePatterns": ["test/**/*.js"]
+    "overrides": [
+        {
+            "files": ["test/**/*.js"],
+            "env": {
+                "jest": true
+            }
+        }
+    ]
 }