os: use t.TempDir in TestMkdirTemp, TestCreateTemp

This simplifies tests a little bit.

Change-Id: I910e3c97cfd20b26951d2a4909d86b5be06bde56
Reviewed-on: https://go-review.googlesource.com/c/go/+/606899


LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>