Create a separate file for JS-specific unit tests.

Reviewers: djasper

Reviewed By: djasper

CC: cfe-commits, klimek

Differential Revision: http://llvm-reviews.chandlerc.com/D2307

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@196266 91177308-0d34-0410-b5e6-96231b3b80d8