Fix flaky spec

Seems like StackProf.results is leaking between specs
when it's stubbed via `allow`, so we gotta restub it
in the profiler spec too.

sentry-ruby/spec/sentry/profiler_spec.rb

@@ -174,10 +174,11 @@
         subject.set_initial_sample_decision(true)
         subject.start
         subject.stop
+
+        allow(StackProf).to receive(:results).and_return([])
       end
 
       it 'returns empty' do
-        expect(StackProf).to receive(:results).and_call_original
         expect(subject.to_hash).to eq({})
       end