Use reflection to find/load/use MonoMac.dll

We don't want our test runner to compile against a particular version
of MonoMac. We want it to be able to run/execute any version of MonoMac
or Xamarin.Mac.

As such use reflection to invoke the handful of methods we require and
assume that whatever test assembly we are loading loads up the required
MonoMac or Xamarin.Mac assembly so Type.GetType is able to find NSApplication.

NOTE: We only look for MonoMac now. It'd be trivial to also look for
Xamarin.Mac prefixed types.