selftests: forwarding: mirror_vlan: Change test description

The test description is displayed with the PASS/FAIL resolution after
the test is ran. There however already is one other test described
exactly like this, which makes it unclear which of the tests passed or
failed. Make the description unique.

Signed-off-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>