Add test with defmt feature.

.github/workflows/test.yml

@@ -70,6 +70,7 @@ jobs:
         features:
           # These feature sets cannot run tests, so we only check they build.
           - medium-ip medium-ethernet proto-ipv6 proto-ipv6 proto-igmp proto-dhcpv4 socket-raw socket-udp socket-tcp socket-icmp async
+          - defmt medium-ip medium-ethernet proto-ipv6 proto-ipv6 proto-igmp proto-dhcpv4 socket-raw socket-udp socket-tcp socket-icmp async
 
     steps:
       - uses: actions/checkout@v2