Added optional callback testReady(test).

The testReady(test) is called before a test function is run with
the same test object that will be handed to the test function.
This allows for decorating the test object with additional
functionality.

For instance, in the case of
http://github.com/dalnefre/tart-nodeunit it enables adding
test.sponsor(...) and test.dispatch(...) without changing nodeunit.