ensure pipeline terminates execution before doing assertion

This test relies on the generator input sending 10 events and observing
the output. Calling shutdown immediately after start can cause the
plugin to abort earily.
This change waits for the execution to finish before performing the
test assertions.

Author: jsvd

logstash-core/spec/logstash/java_pipeline_spec.rb

@@ -903,6 +903,7 @@ def flush(options)
     it "correctly distributes events" do
       pipeline = mock_java_pipeline_from_string(config, pipeline_settings_obj)
       pipeline.start
+      sleep 0.01 until pipeline.finished_execution?
       pipeline.shutdown
       expect(output.events.size).to eq(60)
       expect(output.events.count {|e| e.get("cloned") == "cloned"}).to eq(30)