Add script for auto-re-running tests on change

`npm run watch:test:unit` will re-run the unit test suite whenever a
file in `app`, `ui`, or `test` changes.

package.json

@@ -9,6 +9,7 @@
     "dist": "gulp dist",
     "doc": "jsdoc -c development/tools/.jsdoc.json",
     "test": "npm run test:unit && npm run test:integration && npm run lint",
+    "watch:test:unit": "nodemon --exec \"npm run test:unit\" ./test ./app ./ui",
     "test:unit": "cross-env METAMASK_ENV=test mocha --exit --require test/setup.js --recursive \"test/unit/**/*.js\" \"ui/app/**/*.test.js\" && dot-only-hunter",
     "test:single": "cross-env METAMASK_ENV=test mocha --require test/helper.js",
     "test:integration": "npm run test:integration:build && npm run test:flat && npm run test:mascara",