Fix tinytest invocation from windows shell

Fix tinytest invocation from windows shell

Original post:

  This post is in response to a posting last December on a Windows
  regression fork failure ([Libevent-users] Re: Libevent 2.0.10-stable
  is released by Dongsheng Song).  I noticed the question was not
  answered and I recently experienced the same error myself when
  trying to run the Windows regression tests myself.

  I checked the return status from the CreateProcess call and found it
  was "file not found".  This led me to look at the command-line I was
  using which was .\regress in a Visual Studio 2008 command prompt
  window.  Windows could not find the file because it did not have the
  .exe extension on the end.  The code that builds the command should
  be modified to ensure the extension is present.