selftests: forwarding: mirror_gre_vlan_bridge_1q: Rename two tests

Rename test_gretap_forbidden() and test_ip6gretap_forbidden() to a more
specific test_gretap_forbidden_cpu() and test_ip6gretap_forbidden_cpu().
This will make it clearer which is which when further down a patch is
introduced that forbids a VLAN on regular bridge port.

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