cmd/link: fix TestUnresolved in module mode

Updates #30228

Change-Id: I9f0e7e59922bd56b17889f72124b7d14b2433218
Reviewed-on: https://go-review.googlesource.com/c/162833
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>

src/cmd/link/link_test.go

@@ -139,6 +139,7 @@ func TestUnresolved(t *testing.T) {
 	// linker would find an undefined reference to "zero" created
 	// by the runtime package.
 
+	write("go.mod", "module testunresolved\n")
 	write("main.go", `package main
 
 func main() {