DO NOT MERGE THIS

[test] Port mdtool run-md-tests to use GuiUnit

This allows us to
1) Use async tests, if we want.
2) Have our tests run inside the Gtk main loop (if we want)
3) Get updates on the fly as the tests execute

As we control GuiUnit we can add additional Assert methods to
keep API compatibility with nunit 2.6.x if we do not actually
want to change our tests to cope with the new api.

This is just to demonstrate the changes required.