tests: section config: comment need for warn on debugging

tests: section config: comment need for warn on debugging

as often only warn really makes it out of perl/our pit of std out/err
handling (e.g., I had a case where neither print STDERR nor syslog
worked, but warn did)

also, the tests are rather brittle w.r.t their expect_fail variant,
as the actual expected error should be enforced.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>