[Modules] Correct test comment from obsolete earlier version of code. NFC

Summary:
Code committed in rL290219 went through a few iterations; test wound up with
stale comment.

Reviewers: doug.gregor, manmanren

Reviewed By: manmanren

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D28790

llvm-svn: 292435