internal/testtext: introduce backward compatible subtest Run

Change-Id: I74a1b5a51cc2e83773ac99ba916c84c6fbeff3dd
Reviewed-on: https://go-review.googlesource.com/23900
Run-TryBot: Marcel van Lohuizen <mpvl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Nigel Tao <nigeltao@golang.org>
3 files changed