Expand systemd e2e tests

With tests for sandboxing, multiuser, and dockerd:

- Exec sandboxing knobs: PrivateTmp, ProtectSystem=strict, User=, and
  NoNewPrivileges.
- Multiuser operation: two users with their own service managers,
  daemons, journals, and delegated cgroups, isolated from each other.
  User commands run through systemd-machined, which resolves users
  created inside the sandbox (docker exec --user cannot).
- TestSystemdDocker now runs a container instead of stopping at pull.

These tests rely on the two preceding fixes: the NoNewPrivileges probe
reads the new NoNewPrivs line in /proc/[pid]/status, and user-unit
sandboxing fails unless mountinfo shows nodev.

PiperOrigin-RevId: 970106945