Fix a bug that would cause genty to fail when trying to expand tests def...

...ined by the metaclass.

delattr(cls, attr) fails when attr is defined by the metaclass with AttributeError. Just catch that error and pass.
This is a slightly simplified version of a bug I had to fix to get genty to work in the sync codebase.

I'm open to suggestions for something simpler, but this is definitely in real code.