[libcxx][filesystem] Remove setgid from parent before testing permissions

[Hahnfeld]

Summary

man page for mkdir says: If the parent directory has the set-group-ID bit set, then so will the newly created directory.

[EricWF]

LGTM. Do you need me to commit this?

test/std/experimental/filesystem/fs.op.funcs/fs.op.create_directory/create_directory_with_attributes.pass.cpp

TEST_CASE(create_existing_directory)
 
     // Test throwing version
     TEST_CHECK(fs::create_directory(dir, dir2) == false);
 }
 
 TEST_CASE(create_directory_one_level)
 {
     scoped_test_env env;
+    // Remove setgid which mkdir would inherit
+    permissions(env.test_root, perms::remove_perms | perms::set_gid);
+
     const path dir = env.make_env_path("dir1");
     const path attr_dir = env.create_dir("dir2");
     permissions(attr_dir, perms::none);
 
     std::error_code ec;
     TEST_CHECK(fs::create_directory(dir, attr_dir, ec) == true);
     TEST_CHECK(!ec);
     TEST_CHECK(is_directory(dir));