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>