cmd/go: add a Go source file in TestScript/mod_sumdb

This test expects 'go mod tidy' to fail if the existing module graph
has a bad checksum. However, there is no intrinsic reason why 'go mod
tidy' should fail in that case: the module contains no packages, and
thus no imports, so 'go mod tidy' can justifiably remove all
requirements without regard to any errors that may have already been
present in the module graph.

Adding a source file that imports a package from the module with the
bad checksum should guarantee that 'go mod tidy' reports the checksum
eror.

For #36460

Change-Id: I59734ac524031288bc03a11f58eed5abe2db76b3
Reviewed-on: https://go-review.googlesource.com/c/go/+/309334


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