Networkd tests: add bridge link properties to test

Networkd tests: add bridge link properties to test

Test the "[Bridge]" section keys

```
[Bridge]
UnicastFlood=true
HairPin=true
UseBPDU=true
FastLeave=true
AllowPortToBeRoot=true
Cost=555
Priority=23
```

```
test_bridge_init (__main__.BridgeTest) ... ok
test_bridge_port_priority (__main__.BridgeTest) ... ok
test_bridge_port_priority_set_zero (__main__.BridgeTest)
It should be possible to set the bridge port priority to 0 ... ok
test_bridge_port_property (__main__.BridgeTest)
Test the "[Bridge]" section keys ... ok

```