Fix logic in Api::Internal test

The intention of this test is to ensure that the service class
MergeRequests::PushOptionsHandlerService does not run when the
:mr_push_options feature flag is disabled.

This test was passing, however was not testing what it was supposed to
be!

For one, setting Feature.disable(:feature) in the test does not disable
the feature, as rspec config in spec_helper stubs Feature to make all
features enabled:

https://gitlab.com/gitlab-org/gitlab-ce/commit/3ee48e422defaedd69946c607bd8d3672e510375

So the feature was still enabled in the test.

But this test wasn't failing because unfortunately I had put:

```
expect(MergeRequests::PushOptionsHandlerService).to receive(:new)
```

instead of not_to!

This meant that the `.new` method was being stubbed, so the service
class did not create a MergeRequest, which satisfied the second
expectation.

```
expect(MergeRequests::PushOptionsHandlerService).to receive(:new)
```

spec/requests/api/internal_spec.rb

@@ -998,15 +998,13 @@ describe API::Internal do
 
       context 'when the feature is disabled' do
         it 'does not invoke MergeRequests::PushOptionsHandlerService' do
-          Feature.disable(:mr_push_options)
+          stub_feature_flags(mr_push_options: false)
 
-          expect(MergeRequests::PushOptionsHandlerService).to receive(:new)
+          expect(MergeRequests::PushOptionsHandlerService).not_to receive(:new)
 
           expect do
             post api('/internal/post_receive'), params: valid_params
           end.not_to change { MergeRequest.count }
-
-          Feature.enable(:mr_push_options)
         end
       end
     end