reintroduce AsyncTransport spec while skipping jruby

spec/datadog/tracing/workers_spec.rb

@@ -72,4 +72,36 @@
       expect(worker.start).to be nil
     end
   end
+
+  describe '#stop' do
+    before { skip if PlatformHelpers.jruby? } # DEV: this test causes jruby-9.2 to fail
+
+    it 'stops underlying thread with default timeout' do
+      expect_any_instance_of(Thread).to receive(:join).with(
+        Datadog::Tracing::Workers::AsyncTransport::DEFAULT_SHUTDOWN_TIMEOUT
+      ).and_call_original
+
+      worker.start
+      worker.stop
+    end
+
+    context 'with shutdown timeout configured' do
+      let(:worker) do
+        described_class.new(
+          transport: nil,
+          buffer_size: 100,
+          on_trace: task,
+          interval: 0.5,
+          shutdown_timeout: 1000
+        )
+      end
+
+      it 'stops underlying thread with configured timeout' do
+        expect_any_instance_of(Thread).to receive(:join).with(1000).and_call_original
+
+        worker.start
+        worker.stop
+      end
+    end
+  end
 end