fix(test): assign fixed address to bridge

Kernel uses the lowest MAC of all attached ports as MAC address of the
bridge. However, after the first port gets attached, DHCP can be started on the
bridge and it will use the MAC (and client-id) of the first port; so, the MAC
used for DHCP depends on the order of attachment of ports, which is not
guaranteed to be stable in general.

To make sure that the DHCP server always sees the request from the MAC of
enp0s1, assign that MAC to the bridge from the kernel command line.

test/TEST-50-MULTINIC/test.sh

@@ -169,7 +169,7 @@ test_client() {
     # bridge, where only one interface is actually connected
     client_test "MULTINIC bridging" \
         00 01 02 \
-        "root=nfs:192.168.50.1:/nfs/client ip=bridge0:dhcp  bridge=bridge0:enp0s1,enp0s5,enp0s6" \
+        "root=nfs:192.168.50.1:/nfs/client ip=bridge0:dhcp::52:54:00:12:34:00 bridge=bridge0:enp0s1,enp0s5,enp0s6" \
         "bridge0" || return 1
     return 0
 }