testing: explain using a _test package
The existing documentation did not explain the difference between
placing a _test.go file in the same package as what is being
tested vs. adding it to a separate _test package. This explains the
distinction and adds an example.
Concept is explained well here: https://stackoverflow.com/a/
31443271
Fixes #25223
Change-Id: Iebaba15207d8aa24f0b370d8dd4062eadb504b5c
GitHub-Last-Rev:
7f49c5f4624b358af8052272da8ac3240751ada0
GitHub-Pull-Request: golang/go#54160
Reviewed-on: https://go-review.googlesource.com/c/go/+/420415
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Heschi Kreinick <heschi@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>