Simplify `simpleRequire`

[tbosch]

Define a module like this:

 uitest.define('someModule', function(require) {
     var otherModule = require('someOtherModule');
 }

A module can load dependent modules using the given require argument in the callback.
Every require function has a cache behind it.

A require function can be created using uitest.requireFactory(parentRequire), optionally specifying a parent require function.

This should simply the code of simpleRequire.js a lot, and the parent relationship should also prevent double instantiation of non run modules when they are used by run modules (those under the run folder), which get instantiated for every test anew.