tests: Check the existance of WHY-OVS.md instead of WHY-OVS.

WHY-OVS has been renamed to WHY-OVS.md. Update the filenames in run-oftest
and run-ryu scripts

Signed-off-by: Wang Sheng-Hui <shhuiw@gmail.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>