Simple test functionality

The test function is passed to `exercises/common.js`, e.g.:

    require('../common')({
      test: function(done) {
        var passed = true
        var error = null

        done(error, passed)
      }
    })

You can use `lib/match-fbo.js` for simple tests: it uses
gl-fbo-matching and the FBOs created by gl-compare to
check if the output being generated by both shaders is
equivalent. lesson-0 is using this as an example, but if
you'd like something more specific you can pass in your own
function too.
merged in testing code for intro-1