[tlbuild] Some weird problems in test cases

[Karl Berry]

Hi Mojca,

    with a special user that otherwise has no shell

I expect that is the problem.  I can't think of any viable solution.  If
you can't get a normal user, just have to ignore these failures or not
bother to run the tests (or if you feel like spending time trying to
figure out a feasible change, so much the better).

    FAIL: tests/bibtex-openout-test
    ...
    perl: no user 500

These are the few tests I wrote in Perl.  They all use
source/texk/tests/common-test.pl.  All that does is fork a subprocess
(calls system(...)) and read/writes a file.  I suspect the fork fails.
Probably something like getpwuid(500) is failing.

I suppose those tests could be rewritten in sh, but that's not something
I'm going to spend time on ... -k