tests: allow running all the services with SYSTEMD_LOG_LEVEL

tests: allow running all the services with SYSTEMD_LOG_LEVEL

It should make it easier to figure out what exactly services do there.
For example, with SYSTEMD_LOG_LEVEL=debug userdbd (v249) prints
```
varlink-5: New incoming message: {"method":"io.systemd.UserDatabase.GetUserRecord","parameters":{}}
```
before it crashes and systemd-resolved prints
```
varlink-21: New incoming message: {"method":"io.systemd.Resolve.ResolveAddress","parameters":{"address":[127,0,0,1],"flags":0,"ifindex":1000000,"family":0}}
```
and those messages are helpful (especially when scripts causing them
aren't clever enough to keep track of random stuff they send to systemd
:-))