Resolve test errors after dependency updates

Jest updated the JSDOM dependency to 11.12+ in a recent update. This
update causes Jest to throw a security error about localStorage when the
tests are assumed to run in the browser and no `testURL` is defined.

Since our tests are not run in the browser, a workaround for this issue
is to explicitly define the environment as `node`. Hopefully this
protects us from weird edge cases in the future.

Refs: jestjs/jest#6766, jsdom/jsdom#2304

package.json

@@ -39,6 +39,9 @@
     "jest": "^24.5.0",
     "sinon": "^4.4.8"
   },
+  "jest": {
+    "testEnvironment": "node"
+  },
   "keywords": [
     "chrome devtools",
     "cli",