run-tests: partially backout PYTHON quoting

In 7f8b7a060584, I quoted this to support python being installed to
"Program Files".  Even though the string passed to os.popen() is this:

    "c:/Python27/python.exe" -c "import mercurial; print (mercurial.__path__[0])"

... cmd.exe is trying to run this:

    'c:/Python27/python.exe" -c "import'

This caused test-hghave.t to fail, reporting 'unexpected mercurial lib: ""',
because the failed execution prints nothing to stdout.  Py3 fails as though it's
not quoted.  For whatever reason, print() shows up in the output when run with
py2, but not py3, so I'm having a hard time debugging this.  For now, let's fix
the buildbot.

tests/run-tests.py

@@ -3082,7 +3082,7 b' class TestRunner(object):'
         if self._hgpath is not None:
             return self._hgpath
 
-        cmd = b'"%s" -c "import mercurial; print (mercurial.__path__[0])"'
+        cmd = b'%s -c "import mercurial; print (mercurial.__path__[0])"'
         cmd = cmd % PYTHON
         if PYTHON3:
             cmd = _strpath(cmd)