net: stmmac: selftests: Add tests for VLAN Perfect Filtering

Add two new tests for VLAN Perfect Filtering. While at it, increase a
little bit the tests strings lenght so that we can have more descriptive
test names.

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