Add existing unit tests to Xcode project

Summary:
This adds gtest test files to the Xcode project which were
previously only in the cmake config. This is the first of several
planned merges.

Reviewers: beanz, spyffe, jingham

Reviewed By: jingham

Subscribers: lldb-commits

Differential Revision: https://reviews.llvm.org/D36485

llvm-svn: 310417