os: add a test case of copying a file itself via io.Copy
Change-Id: Ib9746cb4f27625cb22620271b280d2da242b2fba
Reviewed-on: https://go-review.googlesource.com/c/go/+/428437
Run-TryBot: Ian Lance Taylor <iant@google.com>
Run-TryBot: Andy Pan <panjf2000@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org> Reviewed-by: Michael Knyszek <mknyszek@google.com> Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>