It's always visible:
$ sudo modprobe sit
$ sudo unshare -n ip l
1: lo: <LOOPBACK> mtu 65536 qdisc noop state DOWN mode DEFAULT group default qlen 1000
...
2: sit0@NONE: <NOARP> mtu 1480 qdisc noop state DOWN mode DEFAULT group default qlen 1000
...
grep already indicates if it matched anything by return value.
Additional advantage is then that if the test fails, the unexpected
matching lines are visible in the log output.
Also fix one case where the presence of a newline was used to generate
an invalid environment assignment.
Tested: with mkosi, which builds the local tree and run ninja tests.
The unit files for test-execute are named like
`exec-(setting-name-in-lower-character)-(optional-text).service`.
However, test units for AmbientCapabilities= are not following this.
So, let's rename them for the consistency.
This does not change anything in the functionality of the test.
The nobody user/group may not synthesized by systemd.
To run tests the functionalities in such situation, this adds tests
by user/group by daemon, as it is expected to exists all environments.
We currently look for "nobody" and "nfsnobody" when testing groups, both
of which do not exist on Ubuntu, our main testing environment. Let's
extend the tests slightly to also use "nogroup" if it exists.
