FSFS is the default filesystem for some time now, update the Windows test  driver to default to FSFS. 
FSFS is the default filesystem for some time now, update the Windows test 
driver to default to FSFS. 

* win-tests.py
  (_usage_exit): change help text for the -f option.
  (global): if -f option isn't specified, default to 'fsfs'.

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