Remove use of any_instance_of in runner spec

In EE we prepend a module into BuildFinishedWorker. This would cause
this spec to fail because RSpec doesn't support the use of
"any_instance_of" with classes that include a prepended module.

To work around this we just stub the "perform_async" class method
instead of stubbing the "perform" instance method.

spec/requests/api/runner_spec.rb

@@ -700,7 +700,7 @@ describe API::Runner do
 
       context 'when tace is given' do
         it 'creates a trace artifact' do
-          allow_any_instance_of(BuildFinishedWorker).to receive(:perform).with(job.id) do
+          allow(BuildFinishedWorker).to receive(:perform_async).with(job.id) do
             CreateTraceArtifactWorker.new.perform(job.id)
           end