Add testcase for cimport+alias+subclass bug

it seems that when I do:
> cimport foo as f
> 
> cdef class DerivedClass(f.BaseClass):
>     pass

I get following compilation errors:
> 3:5: 'BaseClass' is not declared
> 3:5: 'f.pxd' not found

which looks like a bug to me.

When I use plain "cimport foo" + "foo.BaseClass" or "from foo cimport 
BaseClass" + "BaseClass", it works as expected. Cython version 
0.20dev add8091340e (git describe: 0.19-324-gadd8091)

The test may require moving into tests/compile after the bug is fixed.

Mailing list link: TODO