Add testcase for cimport+alias+subclass bug

The test may require moving into tests/compile after the bug is fixed.

tests/run/cimport_alias_subclass.pyx

+cimport cimport_alias_subclass_helper as cash
+
+cdef class Derived(cash.Base):
+    cdef bint foo(self):
+        print "Hello"
+
+def run():
+    """
+    >>> run()
+    Hello
+    """
+    d = Derived()
+    d.foo()

tests/run/cimport_alias_subclass_helper.pxd

+cdef class Base:
+    cdef bint foo(self)