testsuite.at: kill for windows.

testsuite.at: kill for windows.

We use kill to cleanup processes from pidfiles.
Windows has a 'taskkill' which does something similar.
We can check if the process with a PID exists with
'tasklist'. Both tasklist and taskkill return 0 for
both success and failure. So, we will have to grep
to see if there is a o/p.

A typical o/p of tasklist is:
$ tasklist | grep ovs
ovsdb-server.exe              3228 RDP-Tcp#0                  2      6,132 K
ovs-vswitchd.exe              2080 RDP-Tcp#0                  2      5,808 K

$ tasklist //fi "PID eq 3228"

Image Name                     PID Session Name        Session#    Mem Usage
========================= ======== ================ =========== ============
ovsdb-server.exe              3228 RDP-Tcp#0                  2      6,132 K

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