All tests invoked from a single file

Playgrounds are now in their own independent directory, and the tests they were once intertwined with are executed through `tests/index.html`.

Also, all test files no longer bleed their innards into the global namespace (which is a wise idea when you include them all from the same place).

There are a few things I am not entirely happy with - especially the fact that the playgrounds use the test folder's lib subdirectory, but that's something I would not want to solve without consulting others (so do feel free to recommend an appropriate course of action).

P.S. Do yourselves a favor and look at this diff while ignoring whitespace. There's plenty of indentation going on here.