tests: correct misuses of POSIXPERM

tests: correct misuses of POSIXPERM

POSIXPERM requires that a later call to stat(2) (hence "ls -l")
faithfully reproduces what an earlier chmod(2) did.  Some
filesystems cannot satisify this.

SANITY requires that a file or a directory is indeed accessible (or
inaccessible) when its permission bits would say it ought to be
accessible (or inaccessible).  Running tests as root would lose this
prerequisite for obvious reasons.

Fix a few tests that misuse POSIXPERM.

t0061-run-command.sh has two uses of POSIXPERM.

 - One checks that an attempt to execute a file that is marked as
   unexecutable results in a failure with EACCES; I do not think
   having root-ness or any other capability that busts the
   filesystem permission mode bits will make you run an unexecutable
   file, so this should be left as-is.  The test does not have
   anything to do with SANITY.

 - The other one expects 'git nitfol' runs the alias when an
   alias.nitfol is defined and a directory on the PATH is marked as
   unreadable and unsearchable.  I _think_ the test tries to reject
   the alternative expectation that we want to refuse to run the
   alias because it would break "no alias may mask a command" rule
   if a file 'git-nitfol' exists in the unreadable directory but we
   cannot even determine if that is the case.  Under !SANITY that
   busts the permission bits, this test no longer checks that, so it
   must be protected with SANITY.

t1509-root-worktree.sh expects to be run on a / that is writable by
the user and sees if Git behaves "sensibly" when /.git is the
repository to govern a worktree that is the whole filesystem, and
also if Git behaves "sensibly" when / itself is a bare repository
with refs, objects, and friends (I find the definition of "behaves
sensibly" under these conditions hard to fathom, but it is a
different matter).

The implementation of the test is very much problematic.

 - It requires POSIXPERM, but it does not do chmod or checks modes
   in any way.

 - It runs "rm /*" and "rm -fr /refs /objects ..." in one of the
   tests, and also does "cd / && git init --bare".  If done on a
   live system that takes advantages of the "feature" being tested,
   these obviously will clobber the system.  But there is no guard
   against such a breakage.

 - It uses "test $UID = 0" to see rootness, which now should be
   spelled "! test_have_prereq NOT_ROOT"

Signed-off-by: Junio C Hamano <gitster@pobox.com>