fix pipeline spec that didn't wait for pipeline to terminate

this test only called "pipeline.start" and expected an output to receive
the "do_close" method call. Therefore it relies on a race condition that
the pipeline shuts down quickly enough, which could fail.
This change ensures the pipeline fully terminated before making the
assertion

This commit also removes an unused let from the time when logstash
output plugins had workers.

Author: jsvd

logstash-core/spec/logstash/pipeline_spec.rb

@@ -371,20 +371,6 @@ def flush(options)
       eos
     }
 
-    let(:test_config_with_output_workers) {
-      <<-eos
-      input {
-        dummyinput {}
-      }
-
-      output {
-        dummyoutput {
-          workers => 2
-        }
-      }
-      eos
-    }
-
     context "output close" do
       let(:pipeline) { mock_pipeline_from_string(test_config_without_output_workers) }
       let(:output) { pipeline.outputs.first }
@@ -393,13 +379,9 @@ def flush(options)
         allow(output).to receive(:do_close)
       end
 
-      after do
-        pipeline.shutdown
-      end
-
       it "should call close of output without output-workers" do
         pipeline.start
-
+        pipeline.shutdown
         expect(output).to have_received(:do_close).once
       end
     end