[libcxx] [test] Fix running tests on macOS with python3

Summary:
The result of subprocess.check_output() is bytes in python3 which we need
to convert to str(). Simplify this by using the executeCommand() helper.

Reviewers: ldionne, EricWF

Reviewed By: ldionne

Subscribers: christof, libcxx-commits

Differential Revision: https://reviews.llvm.org/D54522

llvm-svn: 346878