Add a basic test that the configured fallback hostname is OK

Ideally, assert_cc() would be used for this, so that it is not possible to even
compile systemd with something like '-Dfallback-hostname=.foo'. But to do a
proper check we need to call hostname_is_valid(), and we cannot depend on being
able to run code (e.g. during cross-compilation). So let's do a very superficial
check in meson, and a proper on in test-util.