Fix tests for Runner#attach_to_execution

These tests were blocking because of the newly introduced
EventLoop. The messages sent to the EventLoop are now mocked
and the EventLoop isn't blocking anymore in the tests.

spec/lib/runner/strategy/docker_container_pool_spec.rb

@@ -264,7 +264,8 @@ describe Runner::Strategy::DockerContainerPool do
     # TODO: add tests here
 
     let(:command) { 'ls' }
-    let(:action) { -> { container_pool.attach_to_execution(command) } }
+    let(:event_loop) { Runner::EventLoop.new }
+    let(:action) { -> { container_pool.attach_to_execution(command, event_loop) } }
     let(:websocket_url) { 'ws://ws.example.com/path/to/websocket' }
   end
 end