Fix VertexAI Unit Test resource handing in SPM (#13310)

Author: paulb777

FirebaseVertexAI/Tests/Unit/vertexai-sdk-test-data/mock-responses

@@ -0,0 +1,4 @@
+Placeholder file for Package.swift - required to prevent a warning.
+
+It should be replaced before running the FirebaseVertexAI unit tests by
+running `scripts/update_vertexai_responses.sh`.

Package.swift

@@ -1312,9 +1312,7 @@ let package = Package(
       dependencies: ["FirebaseVertexAI", "SharedTestUtilities"],
       path: "FirebaseVertexAI/Tests/Unit",
       resources: [
-        // Including this README ensures that SPM will always generate a `Bundle.module`, even if
-        // the mock-responses have not been downloaded with the update_vertexai_responses.sh script.
-        .process("README.md"),
+        .process("vertexai-sdk-test-data/mock-responses"),
       ],
       cSettings: [
         .headerSearchPath("../../../"),