Add missing CleanReturnAsync() to some tests. (#18471)

Content.IntegrationTests/Tests/LogErrorTest.cs

@@ -28,5 +28,7 @@ public sealed class LogErrorTest
         // But errors do
         await server.WaitPost(() => Assert.Throws<AssertionException>(() => logmill.Error("test")));
         await client.WaitPost(() => Assert.Throws<AssertionException>(() => logmill.Error("test")));
+
+        await pairTracker.CleanReturnAsync();
     }
 }