tests: patcher_test: let Popen search for true in PATH

As Edward George reported in https://github.com/eventlet/eventlet/pull/3
There is no /bin/true on Darwin, it is in /usr/bin/true instead.
So we use the fact that Popen() calls execvp(3) which searches for file in PATH.

tests/patcher_test.py

@@ -285,7 +285,7 @@ class Subprocess(ProcessBase):
 eventlet.monkey_patch()
 from eventlet.green import subprocess
 
-subprocess.Popen(['/bin/true'], stdin=subprocess.PIPE)
+subprocess.Popen(['true'], stdin=subprocess.PIPE)
 print "done"
 """
         self.write_to_tempfile("newmod", new_mod)