Two failing tests involving type inference when a type is implicitly con...

...vertible both to and from another type.

Note that the ```List<T>``` case is supposed to resolve without an error, but the ```IEnumerable<T>``` case is supposed to give an error. I do not understand why it is so (perhaps something with generic variance; though I thought that only involved identity conversions), but the behavior is the same in both mcs and csc.