cmd/go: prepare tests for GO111MODULE=on by default

Set GO111MODULE=off explicitly in tests specific to GOPATH mode.

Added a go.mod file to other tests that assumed GOPATH mode.

Fixed an issue in the build metadata file generated in
modload/build.go, which did not end with a newline. This broke the
build_dash_x test, which expects to be able to run the script printed
by 'go build -x' to produce the same result. The script is broken if
the build metadata file doesn't end with a newline.

Change-Id: I59f2a492a9f5a66f6c4aa702f429909d5c5e815d
Reviewed-on: https://go-review.googlesource.com/c/go/+/255051


Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Trust: Jay Conrod <jayconrod@google.com>