Fix the lock errors in the LogStash::Runner test

The test was actually starting an agent with pipeline and the runner has
currently no was to stop the agent. This commit make sure we use an
agent mock instead.

Fix: #6931

Fixes #6957

logstash-core/spec/logstash/runner_spec.rb

@@ -18,6 +18,7 @@ describe LogStash::Runner do
 
   subject { LogStash::Runner }
   let(:logger) { double("logger") }
+  let(:agent) { double("agent") }
 
   before :each do
     clear_data_dir
@@ -34,6 +35,12 @@ describe LogStash::Runner do
     allow(LogStash::Logging::Logger).to receive(:configure_logging) do |level, path|
       allow(logger).to receive(:level).and_return(level.to_sym)
     end
+
+    # Make sure we don't start a real pipeline here.
+    # because we cannot easily close the pipeline
+    allow(LogStash::Agent).to receive(:new).with(any_args).and_return(agent)
+    allow(agent).to receive(:execute)
+    allow(agent).to receive(:shutdown)
   end
 
   after :each do
@@ -57,7 +64,7 @@ describe LogStash::Runner do
       expect(LogStash::Agent).to receive(:new) do |settings|
         expect(settings.get("config.string")).to eq(config)
         expect(settings.get("pipeline.workers")).to eq(20)
-      end
+      end.and_return(agent)
       subject.run("bin/logstash", cli_args)
     end
   end
@@ -72,7 +79,6 @@ describe LogStash::Runner do
 
       before do
         allow(agent).to receive(:shutdown)
-        allow(agent).to receive(:register_pipeline)
       end
 
       it "should execute the agent" do
@@ -390,6 +396,8 @@ describe LogStash::Runner do
       let(:args) { ["--path.settings", "/tmp/a/a/a/a", "-e", "input { generator { count => 1000 }} output {}"] }
 
       it "should not terminate logstash" do
+        # The runner should just pass the code from the agent execute
+        allow(agent).to receive(:execute).and_return(0)
         expect(subject.run(args)).to eq(0)
       end