Goals

• I write my Scala.JS unit tests the same way I already do, and I can continue to run them against Node.JS or Phantom.JS because it's fast. No changes there.
• To run tests against Firefox or Chrome, I simply prefix my SBT command with firefox: or chrome:.
  For example, firefox:test would run tests in Firefox, chrome:testOnly a.b.c would run my a.b.c test in Chrome.
• Testing in FF/Chrome doesn't require recompilation (All in the Land know Scalac is slow). It should use all of the same bits and bobs (especially the output JS) that normal tests use.
• I specify that certain tests will only run in FF and/or Chrome.
  For example, tests that use focus should skip Phantom.JS where I know focus doesn't work.
• I run testAll to run the same tests in all environments (fast-env & Firefox & Chrome). This will happen concurrently.
• I can use FF/Chrome headlessly (i.e. without the windows popping up when the browsers are launched and running).