module-transaction: Add GetUser() method that prompts an user if non-set

We can now finally test this properly both using a mock and through the
interactive module that will do the request for us in various conditions.

cmd/pam-moduler/tests/integration-tester-module/communication_test.go

@@ -29,7 +29,7 @@ func ensureError(t *testing.T, err error, expected error) {
 func ensureEqual(t *testing.T, a any, b any) {
 	t.Helper()
 	if !reflect.DeepEqual(a, b) {
-		t.Fatalf("values mismatch %v vs %v", a, b)
+		t.Fatalf("values mismatch %#v vs %#v", a, b)
 	}
 }