tools/run-tests.py: shlex.split() the value of --command-prefix

BUG=171553

Review URL: https://codereview.chromium.org/12049034

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@13469 ce2b1a6d-e550-0410-aec6-3dcde31c8c00