Merge branch 'jk/sanity'

The tests that wanted to see that file becomes unreadable after
running "chmod a-r file", and the tests that wanted to make sure it
is not run as root, we used "can we write into the / directory?" as
a cheap substitute, but on some platforms that is not a good
heuristics.  The tests and their prerequisites have been updated to
check what they really require.

* jk/sanity:
  test-lib.sh: set prerequisite SANITY by testing what we really need
  tests: correct misuses of POSIXPERM
  t/lib-httpd: switch SANITY check for NOT_ROOT

diff --cc t/test-lib.sh
Simple merge