Fix test crash when using deprecated outputs in the root module (#3249)

Signed-off-by: Christian Mesh <christianmesh1@gmail.com>

internal/command/testdata/test/deprecated_outputs/main.tftest.hcl

@@ -0,0 +1,8 @@
+run "example" {
+  command = plan
+
+  assert {
+    condition = output.example == "example"
+    error_message = "ruh-roh"
+  }
+}