TEST-21-DFUZZER: enable services to be tested

TEST-21-DFUZZER: enable services to be tested

Since e19e17df579823093238edb60470b29a2c5e24a6, timesyncd is disabled by
default, and fuzzing for timesyncd failed.

```
[  754.247451] TEST-21-DFUZZER.sh[658]: Bus: org.freedesktop.timesync1 (system)
[  754.247725] TEST-21-DFUZZER.sh[658]: + systemd-run --pipe --wait -- dfuzzer -b 10000 -n org.freedesktop.timesync1
[  754.282237] TEST-21-DFUZZER.sh[3236]: Running as unit: run-p3236-i3237.service
[  754.348983] TEST-21-DFUZZER.sh[3238]: [SESSION BUS]
[  754.350443] TEST-21-DFUZZER.sh[3238]: Bus not found.
[  754.350705] TEST-21-DFUZZER.sh[3238]: [SYSTEM BUS]
[  754.359249] TEST-21-DFUZZER.sh[3238]: Error while calling method 'GetConnectionUnixProcessID': GDBus.Error:org.freedesktop.DBus.Error.NameHasNoOwner: The connection does not exist
[  754.359520] TEST-21-DFUZZER.sh[3238]: Couldn't get the PID of the tested process
[  754.359764] TEST-21-DFUZZER.sh[3238]: Exit status: 4
[  754.386721] TEST-21-DFUZZER.sh[3236]:           Finished with result: exit-code
[  754.387331] TEST-21-DFUZZER.sh[3236]: Main processes terminated with: code=exited, status=4/NOPERMISSION
[  754.387537] TEST-21-DFUZZER.sh[3236]:                Service runtime: 74ms
[  754.387706] TEST-21-DFUZZER.sh[3236]:              CPU time consumed: 21ms
[  754.389210] TEST-21-DFUZZER.sh[3236]:                    Memory peak: 12.9M (swap: 0B)
```

Also, create dummy interface for networkd and resolved.

Fixes #37834.