Fix sorting of parent directories.

This refactoring reversed the order of the "ParentDirectories" function: https://github.com/GoogleContainerTools/kaniko/pull/1155/commits/ffc372a63bca75e5b7c680c981d2444fc4017897#diff-d36eb675aa49a7b471e3a2be77005b18R465

As a side-effect, parent directories weren't added in lexicographical
order, which broke some tests. We now ensure in unit test that the order
of the ParentDirectories function is stable.

pkg/util/fs_util.go

@@ -468,10 +468,10 @@ func ParentDirectories(path string) []string {
 		}
 		dir, _ = filepath.Split(dir)
 		dir = filepath.Clean(dir)
-		paths = append(paths, dir)
+		paths = append([]string{dir}, paths...)
 	}
 	if len(paths) == 0 {
-		paths = append(paths, config.RootDir)
+		paths = []string{config.RootDir}
 	}
 	return paths
 }

pkg/util/fs_util_test.go

@@ -213,8 +213,6 @@ func Test_ParentDirectories(t *testing.T) {
 			defer func() { config.RootDir = original }()
 			config.RootDir = tt.rootDir
 			actual := ParentDirectories(tt.path)
-			sort.Strings(actual)
-			sort.Strings(tt.expected)
 
 			testutil.CheckErrorAndDeepEqual(t, false, nil, tt.expected, actual)
 		})