Tests that depend on filesystem permissions FAIL if run on Windows or as
root. Add a helper function to detect those cases, so the tests can skip
those checks gracefully.

Signed-off-by: Jonathan Wakely <jwakely@redhat.com>

libstdc++-v3/ChangeLog:

	PR libstdc++/90787
	* testsuite/27_io/filesystem/iterators/directory_iterator.cc:
	Use new __gnu_test::permissions_are_testable() function.
	* testsuite/27_io/filesystem/iterators/recursive_directory_iterator.cc:
	Likewise.
	* testsuite/27_io/filesystem/operations/exists.cc: Likewise.
	* testsuite/27_io/filesystem/operations/is_empty.cc: Likewise.
	* testsuite/27_io/filesystem/operations/remove.cc: Likewise.
	* testsuite/27_io/filesystem/operations/remove_all.cc: Likewise.
	* testsuite/27_io/filesystem/operations/status.cc: Likewise.
	* testsuite/27_io/filesystem/operations/symlink_status.cc:
	Likewise.
	* testsuite/27_io/filesystem/operations/temp_directory_path.cc:
	Likewise.
	* testsuite/experimental/filesystem/iterators/directory_iterator.cc:
	Likewise.
	* testsuite/experimental/filesystem/iterators/recursive_directory_iterator.cc:
	Likewise.
	* testsuite/experimental/filesystem/operations/exists.cc:
	Likewise.
	* testsuite/experimental/filesystem/operations/is_empty.cc:
	Likewise.
	* testsuite/experimental/filesystem/operations/remove.cc:
	Likewise.
	* testsuite/experimental/filesystem/operations/remove_all.cc:
	Likewise.
	* testsuite/experimental/filesystem/operations/temp_directory_path.cc:
	Likewise.
	* testsuite/util/testsuite_fs.h (__gnu_test::permissions_are_testable):
	New function to guess whether testing permissions will work.

Tested x86_64-linux. Committed to trunk.