os: add a test case of copying a file itself via io.Copy
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 <87e9c6d529889242b7e184afb632328636553ab4@google.com>
Run-TryBot: Andy Pan <e1a1450fb68401f791397dc71aecc2069ecda608@gmail.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Michael Knyszek <4473450dc5c88f06e1fb787f303d7805ec2aee65@google.com>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@google.com>
Auto-Submit: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@google.com>

golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go