os: implement CreateTemp

Until tinygo implements fastrand(), use a placeholder RNG here.

src/os/export_test.go

@@ -0,0 +1,9 @@
+// Copyright 2011 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.
+
+package os
+
+// Export for testing.
+
+var ErrPatternHasSeparator = errPatternHasSeparator