Mark extension test runner as executable

Summary: This is the last change needed for the "make test" command to work for me, hopefully.
Closes https://github.com/facebook/hhvm/pull/4420

Reviewed By: @JoelMarcey

Differential Revision: D1730984

Signature: t1:1730984:1418349868:b18b10b490378553552c99ed001c259794d120c8