test-flows: Change the way flows are read.

With Visual Studio and Msys combination, something is going
wrong when we do a '3<pcap' followed by fdopen(3, 'rb'). fdopen
fails. I do not know the exact reason for the failure. But the
workaround is straightforward.

Signed-off-by: Gurucharan Shetty <gshetty@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>
testsuite.at: pids can have zero after first character.

Fix the bug which did not kill the processes with pids that had
a zero in them. Also, some tests do a AT_CHECK([kill `cat pid`]) which
on windows prints something on the stdout causing the tests to fail.
So supress it.

Signed-off-by: Gurucharan Shetty <gshetty@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>