Fix "make distcheck"

make distcheck does not pass on master (not even with !3527 (merged) applied):

system test fails because of read-only directories in the tarball
system test fails all over the place
lib/dns/tests unit tests are not being executed (!3527 (comment 134700))
some unit tests fail (file_test)

Edited Sep 11, 2020 by Michal Nowak