net: stmmac: selftests: Add selftest for VLAN TX Offload

Add 2 new selftests for VLAN Insertion offloading. Tests are for inner
and outer VLAN offloading.

Signed-off-by: Jose Abreu <joabreu@synopsys.com>
Signed-off-by: David S. Miller <davem@davemloft.net>