Not all distros support booting without an initrd. E.g. the Debian
kernel builds ext4 as a module and so relies on an initrd to
successfully start the QEMU-based images.
This catches errors like "ninja not found", missing programs etc. early,
instead of silently ignoring them and trying to boot a broken VM.
In install_config_files(), allow some distro specific files to be absent
(such as /etc/sysconfig/init).
All test/TEST* but TEST-02-CRYPTSETUP share the same check_result_qemu()
and test_cleanup(), so move them into test_functions and only override
them in TEST-02-CRYPTSETUP.
Also provide a common test_run() which by default assumes that both QEMU
and nspawn tests are run. Particular tests which don't support either
need to explicitly opt out by setting $TEST_NO_{QEMU,NSPAWN}. Do it this
way around to avoid accidentally forgetting to opt in, and to encourage
test authors to at least always support nspawn.
