[tests] Add VPS test harness.

[tests] Add VPS test harness.

Create tests/vps-base.scm as the framework for Guix system tests.
It defines the test OS (marionette-operating-system wrapping
%simple-os with the VPS services), VM configuration (port forwarding
2222->72), SSH session helper (call-with-connected-session), and the
test harness (run-vps-test) that splices test case gexps into one
derivation.

The SSH config is inlined from conf/vps/sshd.scm with a test
authorized key injected.  %test-vps starts with an empty test case
list -- test cases are added in subsequent commits.

Co-Authored-By: Claude <noreply@anthropic.com>