[release-branch.go1.13] cmd/go: don't split internal test main packages twice

Fixes #34328

Change-Id: Ia6253038c525089e20a1da64a2c5c9dcc57edd74
Reviewed-on: https://go-review.googlesource.com/c/go/+/195677


Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
(cherry picked from commit 4d18a7ce)
Reviewed-on: https://go-review.googlesource.com/c/go/+/197064
Run-TryBot: Bryan C. Mills <bcmills@google.com>