test-execute: add tests with user/group daemon

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.

test/test-execute/exec-user.service

@@ -1,7 +1,7 @@
 [Unit]
-Description=Test for User
+Description=Test for User (daemon)
 
 [Service]
-ExecStart=/bin/sh -x -c 'test "$$USER" = "nobody"'
+ExecStart=/bin/sh -x -c 'test "$$USER" = "daemon"'
 Type=oneshot
-User=nobody
+User=daemon