cmd/link: fix TestStrictDup in module mode

Fixes #35790

Change-Id: Ie9df103c5c21b34a378821dfad60804ce4b0cad4
Reviewed-on: https://go-review.googlesource.com/c/go/+/208517Reviewed-by: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/link/link_test.go

@@ -420,6 +420,11 @@ func TestStrictDup(t *testing.T) {
 	if err != nil {
 		t.Fatal(err)
 	}
+	src = filepath.Join(tmpdir, "go.mod")
+	err = ioutil.WriteFile(src, []byte("module teststrictdup\n"), 0666)
+	if err != nil {
+		t.Fatal(err)
+	}
 
 	cmd := exec.Command(testenv.GoToolPath(t), "build", "-ldflags=-strictdups=1")
 	cmd.Dir = tmpdir