Add basic swarming UI tests

appengine/swarming/ui/README.md

 This contains all the Polymer 1.X elements used in swarming.
 
 To clean and build the pages for deploying, run
 
     npm install   # Need to only run once to set up dev dependencies.
     make vulcanize
 
 This combines all of the elements needed to display the page into several
 "single-page" apps, like the bot-list.
 These are checked into version control so that they may be easily redeployed w/o
@@ skipping 13 matching lines @@
 To run appengine locally without vulcanizing (preferred debugging mode), run
 
     make debug_local_deploy
 
 
 To access the demo pages on localhost:9050, run
 
     make run
 
 
+To run tests, run
+
+    make test
+
+
 Prerequisites
 =============
 
-You will need to install node.js, npm, and bower, for example:
+You will need to install recent versions of node.js, npm, bower and (optionally)
+web-component-tester:
 
     sudo apt-get install npm nodejs-legacy
     sudo npm install -g bower
+    sudo npm install -g web-component-tester  #if you want to run tests
+
+If you see errors like
+
+    /usr/lib/node_modules/web-component-tester/bin/wct:21
+    promise.then(() => process.exit(0), () => process.exit(1));
+                  ^
+    SyntaxError: Unexpected token )
+
+when running web-component-tester (wct), upgrade node.
 
 
 If you don't want to install npm globally, try
 
     echo prefix = ~/foo/bar >> ~/.npmrc
     curl https://www.npmjs.org/install.sh | sh
     ~/foo/bar/npm install -g bower