selftests: Add IPv6 multicast route generation tests for GRE devices.

selftests: Add IPv6 multicast route generation tests for GRE devices.

The previous patch fixes a bug that prevented the creation of the
default IPv6 multicast route (ff00::/8) for some GRE devices. Now let's
extend the GRE IPv6 selftests to cover this case.

Also, rename check_ipv6_ll_addr() to check_ipv6_device_config() and
adapt comments and script output to take into account the fact that
we're not limited to link-local address generation.

Signed-off-by: Guillaume Nault <gnault@redhat.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Link: https://patch.msgid.link/65a89583bde3bf866a1922c2e5158e4d72c520e2.1752070620.git.gnault@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>