Adds a regression test for logplayback fix in gazebosim#2611 (gazebosim#2619)

* Adds a regression test for logplayback

gazebosim/gazebo_test_cases#1640

This commit adds a really simple regression check to make sure we
actually publish on the right topic.

Signed-off-by: Arjo Chakravarty <arjoc@intrinsic.ai>

* typo fix

Signed-off-by: Arjo Chakravarty <arjoc@intrinsic.ai>

* Make atomic

Signed-off-by: Arjo Chakravarty <arjoc@intrinsic.ai>

---------

Signed-off-by: Arjo Chakravarty <arjoc@intrinsic.ai>
Co-authored-by: Addisu Z. Taddese <addisu@openrobotics.org>

Author: arjo129

test/integration/log_system.cc

@@ -774,6 +774,18 @@ TEST_F(LogSystemTest, GZ_UTILS_TEST_DISABLED_ON_WIN32(RecordAndPlayback))
   // Start server
   Server playServer(playServerConfig);
 
+  // Simulate a client
+  gz::transport::Node node;
+  std::atomic<std::size_t> numMsgs = 0;
+  std::function<void(const msgs::SerializedStepMap &)> mockClient =
+    [&](const msgs::SerializedStepMap &/*_res*/)
+  {
+    numMsgs++;
+  };
+  using namespace std::placeholders;
+  EXPECT_TRUE(node.Subscribe(
+    "/world/default/state", mockClient));
+
   // Callback function for entities played back
   // Compare current pose being played back with the pose from the stateMsg
   test::Relay playbackPoseTester;
@@ -837,6 +849,9 @@ TEST_F(LogSystemTest, GZ_UTILS_TEST_DISABLED_ON_WIN32(RecordAndPlayback))
   // checked in the playbackPoseTester
   playServer.Run(true, 500, false);
 
+  // The client should have received some messages.
+  EXPECT_NE(numMsgs, 0);
+
   // Count the total number of state messages in the log file
   int nTotal{0};
   for (auto it = batch.begin(); it != batch.end(); ++it, ++nTotal) { }