Add NPM command to watch and rebuild tests.

package.json

@@ -33,7 +33,7 @@
     "build-closure": "rollup -c && java -jar utils/build/compiler/closure-compiler-v20160713.jar --warning_level=VERBOSE --jscomp_off=globalThis --jscomp_off=checkTypes --externs utils/build/externs.js --language_in=ECMASCRIPT5_STRICT --js build/three.js --js_output_file build/three.min.js",
     "dev": "rollup -c -w",
     "lint": "eslint src",
-    "test": "echo \"Error: no test specified\" && exit 1"
+    "test": "rollup -c test/rollup.unit.config.js -w"
   },
   "keywords": [
     "three",