For completeness of the archives, here is the solution of my problem.
Taking Joerg's hint I went straight to his website and downloaded the
original sources, build with -g, and checked what was going on. Turned
out mkisofs is looking for a subdirectory named "VIDEO_TS", and the
obvious solution is to change the command line to:
% mkisofs -dvd-video -o /dev/null -v -v test_dvd/
Now it works. Here is an idea: add a message complaining about the
missing VIDEO_TS subdirectory. For some reason there are numerous dvd
howto guides on the internet that advice the commandline
% growisofs -Z /dev/dvd -dvd-video dvd/VIDEO_TS
which I now found out to be wrong (at least with current versions of
mkisofs).
So can you now burn a working DVD?