[python] [tests] Fix test_linkage for unique external linkage

Starting with r314037, anonymous namespaces no longer give
unique-external linkage to variables. However, this linkage can still be
achieved by using a type which is not exterally visible,
e.g. through being declared in an anonymous namespace but used outside
it. Fix the test to take advantage of that.

Differential Revision: https://reviews.llvm.org/D39810

llvm-svn: 317986