The File.WriteAt doesn't verify that the file offset is not changed
when calling WriteAt, although it is what users expect.
Add some new tests to verify that this behavior doesn't regress.
Change-Id: Ib1e048c7333d6efec71bd8f75a4fa745775306f9
Reviewed-on: https://go-review.googlesource.com/c/go/+/656355 Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Ian Lance Taylor <iant@google.com>