FIX: Observe unlisted topic creation restrictions in post creator specs (#19283)

Update failing spec which previously used non-staff user to create
hidden posts.

Also add new spec for non-staff use cases to prevent future
regressions.

spec/lib/post_creator_spec.rb

@@ -41,9 +41,9 @@ RSpec.describe PostCreator do
       expect(post.wiki).to eq(true)
     end
 
-    it "can be created with a hidden reason" do
+    it "creates post with a hidden reason for staff user" do
       hri = Post.hidden_reasons[:flag_threshold_reached]
-      post = PostCreator.create(user, basic_topic_params.merge(hidden_reason_id: hri))
+      post = PostCreator.create(admin, basic_topic_params.merge(hidden_reason_id: hri))
       expect(post.hidden).to eq(true)
       expect(post.hidden_at).to be_present
       expect(post.hidden_reason_id).to eq(hri)
@@ -52,6 +52,16 @@ RSpec.describe PostCreator do
       expect(post.user.post_count).to eq(0)
     end
 
+    it "fails to create post with a hidden reason for non-staff user" do
+      hri = Post.hidden_reasons[:flag_threshold_reached]
+
+      expect do
+        post = PostCreator.create(user, basic_topic_params.merge(hidden_reason_id: hri))
+
+        expect(post).to be_nil
+      end.not_to change { Post.count }
+    end
+
     it "ensures the user can create the topic" do
       Guardian.any_instance.expects(:can_create?).with(Topic, nil).returns(false)
       expect { creator.create }.to raise_error(Discourse::InvalidAccess)