This enables users to instantiate Viewers/Modelers via
```
new Viewer({
additionalModules: [ ... ]
});
```
to bootstrap their own modules with the viewer without overriding the
existing ones.
This commit cleans up reminders of the node-js test suite removal
by removing the test/node folder.
The test structure has been aligned by moving all tests from
* test/spec/browser to test/spec
* test/spec/integration to test/integration