Use t.Chdir and t.TempDir to simplify test case code.

Change-Id: If4de06d2373100ce53a5b8f6702d5f4866e3b23d
Reviewed-on: https://go-review.googlesource.com/c/go/+/606897


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