Add cmd/start_test.go and systemd/start_stop_test.go with regression
tests covering:
- Start/Stop require exactly one unit name argument (cobra.ExactArgs(1)
semantics verified via ValidateArgs)
- Start/Stop build correct exec commands per runtime
- StartAll sorts units by Order asc, then Name asc as tiebreaker
- StopAll sorts units by Order desc, then Name desc as tiebreaker
- StartAll skips disabled units (Enabled=false)
- StopAll skips uninstalled units
These tests enforce the CLI semantics that 'start' starts only a specific
unit and 'stop' stops only a specific unit; 'startall' and 'stopall'
operate on all matching units.
Run: make test
