cmd/go: add a source file in the multiple-paths module in TestScript/mod_tidy_replace

This situation is analogous to CL 309334: the test expects 'go mod
tidy' to fail due to a module used for more than one path in the build
list, but doesn't actually contain any packages or imports — so no
module is necessarily used at all, and the error only occurs if we
report it prematurely.

For #36460

Change-Id: I5ccecf30f280895eba913a8d62571872b75e710d
Reviewed-on: https://go-review.googlesource.com/c/go/+/312098


Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Michael Matloob <matloob@golang.org>