* gnu/tests/install.scm (gui-test-program): Add a desktop? argument, and pass it to choose-services,
(installation-target-os-for-gui-tests): new procedure,
(installation-target-desktop-os-for-gui-tests): new procedure,
(guided-installation-test): add target-os and desktop? arguments. Use
target-os instead of the previous os variable. Pass desktop? argument to
gui-test-program.
(%test-gui-installed-os): Adapt accordingly,
(%test-gui-installed-os-encrypted): ditto,
(%test-gui-installed-desktop-os-encrypted): new exported variable.