ephemeral: Add test-basic subcommand

[henrywang]

Add bcvk ephemeral test-basic <image> which boots an ephemeral VM and verifies systemd reached a healthy state via systemctl is-system-running. This provides a quick smoke test for bootc container images.

Assisted-by: Claude Code (Opus 4.6)

[gemini-code-assist]

Code Review

This pull request introduces a new ephemeral test-basic subcommand to the bcvk tool, which serves as a smoke test by booting an ephemeral VM and verifying its health using systemctl is-system-running. The update includes the command logic, a corresponding integration test, and comprehensive documentation. Feedback was provided to improve the robustness of the health check by adding the --wait flag to the systemctl command and to remove an unnecessary -- separator from the SSH arguments to ensure consistency with existing command-line behavior.

[henrywang]

PASS [ 23.295s] ( 3/18) integration-tests::integration-tests test_ephemeral_test_basic

[cgwalters]

I know I suggested this as a kind of offhand comment, but a few things here.

• Commands we expose should generally also appear as varlink; maybe going forward we actually try to have the CLI be sugar around varlink just to force this
• If we're making "is system healthy" commands probably it should apply to libvirt too?
• Bigger picture...I would really like bcvk to help be an awesome component of testing frameworks - but without growing into a complete framework itself ideally. But in that vein we need to ask how to make this extensible

Combining the above...one thing I was thinking about here and have some code towards in an unpushed commit is - what if we bridged e.g. the dbus socket or systemd dbus/varlink itself? Worth investigating because then it becomes way more flexible, we aren't just hardcoding one systemd API call.