selftests: forwarding: Fix mausezahn delay parameter in mirror_test()

Currently, mausezahn delay parameter in mirror_test() is specified with
'ms' units.

mausezahn versions before 0.6.5 interpret 'ms' as seconds and therefore
the tests that use mirror_test() take a very long time to complete.

Resolve this by specifying 'msec' units.

Signed-off-by: Danielle Ratson <danieller@mellanox.com>
Reviewed-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>