tests-windows: Make flow extractor work on Windows

+Traceback (most recent call last):
+  File "../.././flowgen.py", line 204, in <module>
+    flows = os.fdopen(3, 'wb')
+OSError: [Errno 9] Bad file descriptor
./library.at:4: exit code was 1, expected 0

Unfortunately 3/4/5/.. FDs don't work on Windows. Switch to a filename open
instead of opening the FDs.

Signed-off-by: Alin Gabriel Serdean <aserdean@ovn.org>
Acked-by: Ben Pfaff <blp at ovn.org>

tests/flowgen.py

@@ -14,7 +14,6 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-import os
 import socket
 import struct
 
@@ -201,8 +200,8 @@ def output(attrs):
     packets.write(packet)
 
 
-flows = os.fdopen(3, 'wb')
-packets = os.fdopen(4, 'wb')
+flows = open('flows', 'wb')
+packets = open('pcap', 'wb')
 
 # Print pcap file header.
 packets.write(struct.pack('>LHHLLLL',

tests/library.at

@@ -1,7 +1,7 @@
 AT_BANNER([library unit tests])
 
 AT_SETUP([flow extractor])
-AT_CHECK([$PYTHON $srcdir/flowgen.py >/dev/null 3>flows 4>pcap])
+AT_CHECK([$PYTHON $srcdir/flowgen.py >/dev/null])
 AT_CHECK([ovstest test-flows flows pcap], [0], [checked 247 packets, 0 errors
 ])
 AT_CLEANUP