Add systemd service files equivalent to xdebian-edu-firstboot init script

The init script performs two tasks on the first boot after installation. Firstly, it starts the run-at-firstboot script from debian-edu-config. Secondly, it runs tests via the debian-edu-test-install script provided by debian-edu-config if requested by the user via the testinstall argument on the kernel command line. Any errors found will be mailed to the first user and success or failure will be signaled through a graphical dialog on top of the display manager. Factor out the test runner into a separate script debian-edu-testsuite. This script is then run by the xdebian-edu-firstboot sysvinit script. Create two systemd service files, one which runs the run-at-firstboot script and another one for running debian-edu-testsuite. Remove unsupported update-rc.d options.

The testsuite has been tested and runs athough there are failing thests which is another separate issue. The gross hack to display a dialog box with the test results on top of the display manager still works with systemd (it hangs the unit in activating state until the dialog is closed but there seems to be no timeout). IMHO we should get rid of this, logs and email notifications are sufficient.