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
:-))
If we assign our own test runner, passing arguments stops working
as unittest won't instantiate its own test runner after it parses
the arguments from sys.argv.
Consequence is that the tests will write to stderr now instead of
stdout since it doesn't seem possible to configure the stream that
unittest.main() will instantiate its test runner with so it'll
default to sys.stderr.
otherwise units using `DynamicUser=yes` won't be able to write the
coverage stats (currently affecting TEST-20-MAINPIDGAMES).
`DynamicUser=yes` implies `ProtectSystem=strict` and
`ProtectHome=read-only` and can't be overridden hence we need to
utilize `ReadWritePaths=` to work around that.
When checking if we look at the root directory we actually need to
compare both st_dev *and* st_ino. The existing check only checked the
latter. Fix that.
