test: performance, remove Popen(shell=True) on Win

not needed according to official python docs -
https://docs.python.org/2/library/subprocess.html#index-2

PR-URL: https://github.com/nodejs/node/pull/12138


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Nikolai Vavilov <vvnicholas@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>