tests: Add extlinux gpt test.

* gnu/tests/install.scm (%test-installed-extlinux-os): New exported
  variable.
(%minimal-extlinux-os, %minimal-extlinux-os-source): New variables.
(%extlinux-gpt-installation-script): New variable.
(run-install)[packages]: New argument. The packages specified are
appended to the installation image.