testing: implement testing.TempDir

TODO: enable test on windows and wasi once os.Readdir and os.RemoveAll are implemented there

src/testing/is_wasi_test.go

@@ -0,0 +1,10 @@
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+//
+//go:build wasi
+// +build wasi
+
+package testing_test
+
+const isWASI = true