testing: add Testing function

testing: add Testing function

The Testing function reports whether the program is a test created
by "go test".

Fixes #52600

Change-Id: Ie0fff7c7dfdfdf997c18b4b6112632600b327cc8
Reviewed-on: https://go-review.googlesource.com/c/go/+/475496
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>