manager: allow test run to catch SIGCHLD events

Otherwise we cannot know when a service exited
tests: add test-execute

add tests for the following directives:
- WorkingDirectory
- Personality
- IgnoreSIGPIPE
- PrivateTmp
- SystemCallFilter: It makes test/TEST-04-SECCOMP obsolete, so it has
  been removed.
- SystemCallErrorNumber
- User
- Group
- Environment