Add rootful Podman integration test cells

Not ready for review - work in progress.

Relates to #31043.

Adds the two rootful cells of the {rootful, rootless} x {root, non-root} Podman test matrix, completing the local opt-in integration tests started in !6991 (merged) (which covered only the rootless cells).

• Adds podmanRootfulHost, parallel to the existing podmanHost, which locates a Linux system-wide (rootful) Podman socket at /run/podman/podman.sock. Skips cleanly on non-Linux hosts, when not running with root privileges, or when the socket isn't reachable.
• Adds TestPodmanRootfulCommandBasicRun and TestPodmanRootfulCommandWithNonRootUser, mirroring the existing rootless tests but pointed at podmanRootfulHost.
• Updates the file's package doc comment to reflect that all four test-matrix cells are now covered, leaving only CI wiring and doc updates outstanding.

Verified against a live rootful Podman 5.8.2 daemon (a root-owned podman.socket inside a Linux VM): both TestPodmanRootfulCommandBasicRun and TestPodmanRootfulCommandWithNonRootUser pass for real, not just via the skip path.

Related follow-up work, tracked in separate MRs and not covered here:

• CI wiring: !7179 (merged)
• Docs update: !7177 (closed)

This MR alone does not close #31043.