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>

tools/test.py

@@ -617,7 +617,6 @@ def RunProcess(context, timeout, args, **rest):
   pty_out = rest.pop('pty_out')
 
   process = subprocess.Popen(
-    shell = utils.IsWindows(),
     args = popen_args,
     **rest
   )