internal/testenv: simplify hasSymlink for windows

internal/testenv: simplify hasSymlink for windows

1. Combine two functions into one.

2. Use errors.Is to check for wrapped errors.

3. Use sync.OnceValues.

Change-Id: I25f55d31bb658ff08da209b1740e9dff579cca69
Reviewed-on: https://go-review.googlesource.com/c/go/+/609797
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Alan Donovan <adonovan@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Commit-Queue: Ian Lance Taylor <iant@golang.org>