[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 4d18a7ceb2d37b148061ee2e153d56aaef4de8fc)
    Reviewed-on: https://go-review.googlesource.com/c/go/+/197064
    Run-TryBot: Bryan C. Mills <bcmills@google.com>