There is a lot of logic in intro.js; now we test four cases:
1. (implicitly, via QUnit tests) A real browser with window being the global
2. Browserify where there are both global & window variables.
3. Node with jsdom where window is passed manually to the jQuery factory.
4. Pure Node with incorrect window passed; jQuery should throw then.
Previously the second & fourth case was not tested and the third was tested
in a way that interfered with the main test environment.
We now also test if in the Browserify case we're not creating a jQuery global
by default.
Fixes gh-2181
Closes gh-2234
