selftests: net: local_termination: introduce new tests which capture VLAN behavior

Add more coverage to the local termination selftest as follows:
- 8021q upper of $h2
- 8021q upper of $h2, where $h2 is a port of a VLAN-unaware bridge
- 8021q upper of $h2, where $h2 is a port of a VLAN-aware bridge
- 8021q upper of VLAN-unaware br0, which is the upper of $h2
- 8021q upper of VLAN-aware br0, which is the upper of $h2

Especially the cases with traffic sent through the VLAN upper of a
VLAN-aware bridge port will be immediately relevant when we will start
transmitting PTP packets as an additional kind of traffic.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>