Add _DBUS_ASSERT_ERROR_XOR_BOOL, and a test for assertions

On !43 (merged), @pwithnall suggested having a macro to encapsulate this pattern:

if (result)
  _DBUS_ASSERT_ERROR_IS_CLEAR (error);
else
  _DBUS_ASSERT_ERROR_IS_SET (error);

This is based on, and includes, !43 (merged).