testing: use temp dir without symlinks in TestChdir/relative

When paths with symlinks are involved, it's not viable to compare them
with string equality. Don't use a temporary directory with symlinks in
it as input, so the test works in more environments.

For #62516.

Change-Id: I95d774365cc2f90eb0ffcffa61229ed5cee43e3e
Reviewed-on: https://go-review.googlesource.com/c/go/+/607035


Reviewed-by: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>