test-ssh: stabilize for Windows

Previously, this complained:

    remote: '.' is not recognized as an internal or external command,
    remote: operable program or batch file.

Making this a python script apparently revealed some races[1].  Thanks to Yuya
for suggesting this.

[1] https://www.mercurial-scm.org/pipermail/mercurial-devel/2017-December/109094.html

tests/test-ssh.t

@@ -603,12 +603,12 b' test that custom environment is passed d'
   > echo \$VAR >&2
   > EOF
   $ chmod +x dumpenv
-  $ hg pull ssh://something --config ui.ssh="./dumpenv"
+  $ hg pull ssh://something --config ui.ssh="sh dumpenv"
   pulling from ssh://something/
   remote: 
   abort: no suitable response from remote hg!
   [255]
-  $ hg pull ssh://something --config ui.ssh="./dumpenv" --config sshenv.VAR=17
+  $ hg pull ssh://something --config ui.ssh="sh dumpenv" --config sshenv.VAR=17
   pulling from ssh://something/
   remote: 17
   abort: no suitable response from remote hg!