[test] Capture stderr from 'tar --version' call as well

Capture the stderr from 'tar --version' call as otherwise error messages
spill onto user's terminal unnecessarily (e.g. on NetBSD where tar does
not support long options).  While at it, refactor the code to use
communicate() instead of reinventing the wheel.

Differential Revision: https://reviews.llvm.org/D55443

llvm-svn: 349204