CI: Make tap0 available

The network-properties test needs this.

.github/workflows/test.yml

@@ -122,6 +122,11 @@ jobs:
             echo
             echo "Testing ${D}"
             echo
+            # Not needed for all tests, but running it always is cheaper than checking.
+            #
+            # Not setting up any particular addresses; tests don't expect any
+            # route so far.
+            ip tuntap add dev tap0 mode tap
             make all test BOARD=native
           fi
         else