test: enable the systemd-resolved unit in TEST-75

Without enabling itx, there's no symlink to the org.freedesktop.resolve1
dbus service, so there exists a tiny window in which the sequence of
`systemctl start` and `systemctl service-log-level` commands might fail:

[ 1127.615151] H systemd[1]: Started Network Name Resolution.
[ 1127.617768] H testsuite-75.sh[34]: + systemctl service-log-level systemd-resolved.service debug
[ 1127.621251] H dbus-daemon[54]: [system] Activating via systemd: service name='org.freedesktop.resolve1' unit='dbus-org.freedesktop.resolve1.service' requested by ':1.24' (uid=0 pid=119 comm="systemctl service-log-level systemd-resolved>
[ 1127.621336] H systemd[1]: dbus-org.freedesktop.resolve1.service: Failed to load configuration: No such file or directory
[ 1127.621364] H systemd[1]: dbus-org.freedesktop.resolve1.service: Trying to enqueue job dbus-org.freedesktop.resolve1.service/start/replace
[ 1127.621395] H systemd[1]: D-Bus activation failed for dbus-org.freedesktop.resolve1.service: Unit dbus-org.freedesktop.resolve1.service not found.
[ 1127.621965] H dbus-daemon[54]: [system] Activation via systemd failed for unit 'dbus-org.freedesktop.resolve1.service': Unit dbus-org.freedesktop.resolve1.service not found.
[ 1127.622046] H systemd[1]: systemd-resolved.service: D-Bus name org.freedesktop.resolve1 now owned by :1.25
[ 1127.622130] H systemctl[119]: Failed to set log level of org.freedesktop.resolve1 to debug: Unit dbus-org.freedesktop.resolve1.service not found.

Spotted in a couple of recent Ubuntu CI runs.

test/units/testsuite-75.sh

@@ -46,7 +46,7 @@ monitor_check_rr() (
 
 # Test for resolvectl, resolvconf
 systemctl unmask systemd-resolved.service
-systemctl start systemd-resolved.service
+systemctl enable --now systemd-resolved.service
 systemctl service-log-level systemd-resolved.service debug
 ip link add hoge type dummy
 ip link add hoge.foo type dummy