all: use t.Chdir in tests

all: use t.Chdir in tests

Change-Id: I5bc514bedeb1155e6db52e37736fd6101774aea0
Reviewed-on: https://go-review.googlesource.com/c/go/+/529896
Auto-Submit: Ian Lance Taylor <iant@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Commit-Queue: Ian Lance Taylor <iant@golang.org>