test/driver: Fix cross-testing on recent Cygwin

Fixes error:

    Traceback (most recent call last):
      File "driver", line 70, in <module>
        _cygwin = ctypes.CDLL('cygwin1', use_errno=True)
      File "/usr/lib/python2.7/ctypes/__init__.py", line 365, in __init__
        self._handle = _dlopen(self._name, mode)
    OSError: No such file or directory

test/driver.in

@@ -66,7 +66,7 @@ SKIP = object()
 if '!!CYGWIN_CROSS_TEST!!':
     import ctypes
 
-    _cygwin = ctypes.CDLL('cygwin1', use_errno=True)
+    _cygwin = ctypes.CDLL('cygwin1.dll', use_errno=True)
     _cygwin_conv_path = _cygwin.cygwin_conv_path
     _cygwin_conv_path.argtypes = [ctypes.c_uint, ctypes.c_void_p,
             ctypes.c_void_p, ctypes.c_size_t]