src: split nacltest.bash into naclmake.bash and keep nacltest.bash

src: split nacltest.bash into naclmake.bash and keep nacltest.bash

Needed by the build system to shard tests. nacl was the last unsharded
builder.

(I considered also adding a -make-only flag to nacltest.bash, but that
wouldn't fail fast when the file didn't exist.)

Updates #15242

Change-Id: I6afc1c1fe4268ab98c0724b5764c67d3784caebe
Reviewed-on: https://go-review.googlesource.com/21851
Reviewed-by: Andrew Gerrand <adg@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>