test: wait for the interface to become routable after reconfiguring

Since 6e8477edd3 TEST-75 started failing with:

[  571.468298] testsuite-75.sh[46]: + for addr in "${DNS_ADDRESSES[@]}"
[  571.468298] testsuite-75.sh[46]: + run delv @fd00:dead:beef:cafe::1 -t A mail.signed.test
[  571.468899] testsuite-75.sh[562]: + tee /tmp/tmp.qKlHPbCCJZ
[  571.469317] testsuite-75.sh[561]: + delv @fd00:dead:beef:cafe::1 -t A mail.signed.test
[  571.501381] testsuite-75.sh[562]: ;; network unreachable resolving 'mail.signed.test/A/IN': fd00:dead:beef:cafe::1#53
[  571.501564] testsuite-75.sh[562]: ;; resolution failed: SERVFAIL
[  571.515457] testsuite-75.sh[46]: + grep -qF '; fully validated' /tmp/tmp.qKlHPbCCJZ

Let's wait for the dns0 interface to become routable again after
re-enabling IPv6 to, hopefully, mitigate this.

test/units/testsuite-75.sh

@@ -29,6 +29,7 @@ disable_ipv6() {
 enable_ipv6() {
     sysctl -w net.ipv6.conf.all.disable_ipv6=0
     networkctl reconfigure dns0
+    /usr/lib/systemd/systemd-networkd-wait-online --ipv4 --ipv6 --interface=dns0:routable --timeout=30
 }
 
 monitor_check_rr() (