[Bug libstdc++/90787] New: filesystem tests fail if file permissions are not supported

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90787

            Bug ID: 90787
           Summary: filesystem tests fail if file permissions are not
                    supported
           Product: gcc
           Version: 10.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: redi at gcc dot gnu.org
  Target Milestone: ---

Running the test as root gives:

FAIL: 27_io/filesystem/iterators/directory_iterator.cc execution test
FAIL: 27_io/filesystem/iterators/recursive_directory_iterator.cc execution test
FAIL: 27_io/filesystem/operations/exists.cc execution test
FAIL: 27_io/filesystem/operations/is_empty.cc execution test
FAIL: 27_io/filesystem/operations/remove.cc execution test
FAIL: 27_io/filesystem/operations/status.cc execution test
FAIL: 27_io/filesystem/operations/symlink_status.cc execution test
FAIL: 27_io/filesystem/operations/temp_directory_path.cc execution test
FAIL: experimental/filesystem/operations/remove.cc execution test
FAIL: experimental/filesystem/operations/temp_directory_path.cc execution test
FAIL: experimental/filesystem/operations/exists.cc execution test
FAIL: experimental/filesystem/operations/is_empty.cc execution test
FAIL: experimental/filesystem/iterators/directory_iterator.cc execution test
FAIL: experimental/filesystem/iterators/recursive_directory_iterator.cc
execution test

This is because those tests assume that it's possible to make a file or
directory unreadable, so that a permission denied error will be given. On
Windows we can't set permissions, and that's probably true on other supported
targets too. The superuser (or any user with suitable privs) can access any
file even if it isn't readable. The testsuite should skip tests that rely on
permissions if they aren't going to work.

That will also entail splitting some test files into the parts that rely on
permissions and the parts that don't.