[ASTImporter] Enable disabled but passing test

Summary:
There is a test which passes since D32947, but it was forgotten to be enabled.
This patch enables that disabled test.

Reviewers: a.sidorin, r.stahl, xazax.hun

Subscribers: rnkovacs, dkrupp, cfe-commits

Differential Revision: https://reviews.llvm.org/D47069

llvm-svn: 332728