command_buffer_event_sync.cpp: add delay for testing clSetEventCallback

according to specification, these can be executed asynchronously
by the OpenCL runtime.

test_conformance/extensions/cl_khr_command_buffer/command_buffer_event_sync.cpp

@@ -18,6 +18,8 @@
 #include "procs.h"
 
 #include <vector>
+#include <thread>
+#include <chrono>
 
 //--------------------------------------------------------------------------
 enum class EventMode
@@ -418,6 +420,11 @@ struct CommandBufferEventSync : public BasicCommandBufferTest
         error = clFinish(queue);
         test_error(error, "clFinish failed");
 
+        for (unsigned i = 0; i < 30; ++i) {
+          std::this_thread::sleep_for(std::chrono::milliseconds(100));
+          if (confirmation) break;
+        }
+
         // verify the result
         if (!confirmation)
         {
@@ -765,6 +772,11 @@ struct CommandBufferEventSync : public BasicCommandBufferTest
         error = clFinish(queue);
         test_error(error, "clFinish failed");
 
+        for (unsigned i = 0; i < 30; ++i) {
+          std::this_thread::sleep_for(std::chrono::milliseconds(100));
+          if (confirmation) break;
+        }
+
         // verify the result
         if (!confirmation)
         {