Make TestPidOf1 unit test more robust

Make TestPidOf1 unit test more robust

The test expects to find PID 1 by searching for "init" or "systemd".

While some LXC container is running:
- PID 1 runs systemd (not traditional init)
- Another process (e.g., PID 41150) may have "init" in its cmdline

This causes pid_of("init") to return PID 41150 instead of PID 1,
and the test would fail because PID 1 is not in the result.

The fix checks if PID 1 is found after the "init" search, and if not,
falls back to searching for "systemd" to find PID 1.