tests: Add gui-uefi-installed-os test.

tests: Add gui-uefi-installed-os test.

* gnu/installer/tests.scm (conclude-installation): Rename it into ...
(start-installation): ... this new procedure.
(complete-installation): New procedure.
(choose-partitioning): Add an uefi-support? argument.
* gnu/tests/install.scm (uefi-firmware): New procedure.
(run-install, qemu-command/writable-image, gui-test-program,
installation-target-os-for-gui-tests): Add an uefi-support? argument.
(%extra-packages): Add grub-efi, fatfsck/static and dosfstools.
(%test-gui-installed-os): New variable.