find_cmd test failure on Windows

[takluyver]

I think this is caused by #3301. The Windows implementation of find_cmd expects a command name without an extension, but the test now uses 'python.exe'.

I think that 'python.exe' is a valid command on Windows, so I think we should modify find_cmd to allow passing a command with an extension. Alternatively, we could modify the test to strip the extension.

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ERROR: Make sure we find sys.exectable for python.
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Traceback (most recent call last):
  File "S:\Users\slave\Jenkins\shiningpanda\jobs\d5f643a2\virtualenvs\ff035a1d\lib\site-packages\nose\case.py", line 197, in runTest
    self.test(*self.arg)
  File "S:\Users\slave\Jenkins\shiningpanda\jobs\d5f643a2\virtualenvs\ff035a1d\lib\site-packages\ipython-1.0.dev-py2.7.egg\IPython\utils\tests\test_process.py", line 36, in test_find_cmd_python
    nt.assert_equal(find_cmd(python), sys.executable)
  File "S:\Users\slave\Jenkins\shiningpanda\jobs\d5f643a2\virtualenvs\ff035a1d\lib\site-packages\ipython-1.0.dev-py2.7.egg\IPython\utils\process.py", line 67, in find_cmd
    raise FindCmdError('command could not be found: %s' % cmd)
FindCmdError: command could not be found: python.exe