that fixes a bunch of tests in pylint.

We also make the synthetic root a singleton, as opposed to a module
   built in the astroid manager. That allows us to use it as a default
   value in Const constructors.

Note the changes to the test. Before, we in fact tested that constants
   don't have a parent. Is that reasonable? In fact, it seems like
   there are a variety of interpretations and we shouldn't commit to
   one by explicitly testing for a certain parent or lack thereof.