[tests] Fix module loading: circular dep, claude env, guix build -e.

[tests] Fix module loading: circular dep, claude env, guix build -e.

- Remove circular import: tests/sshd.scm no longer imports tests/vps-base.
  sshd-test-cases now takes connect-gexp as a parameter.
- Fix conf/home/claude.scm: fall back to getcwd when GUIX_PACKAGE_PATH
  is unset, preventing #f in string-append during auto-compilation.
- Switch Makefile test-vps from guix system test (unavailable in this
  Guix version) to guix build -e.
- Remove home test imports from tests/vps-base.scm for now -- the
  conf/home modules depend on the full Guix environment.

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