[SYCL][NFC] Fix flaky behavior in cleanup related unit tests (#7798)

Author: sergey-semenov

sycl/unittests/scheduler/GraphCleanup.cpp

@@ -301,6 +301,10 @@ TEST_F(SchedulerTest, HostTaskCleanup) {
 
   Lock.unlock();
   Event.wait();
+  // The command should be sent to graph cleanup as part of the task
+  // submitted to the thread pool, shortly after the event is marked
+  // as complete.
+  detail::GlobalHandler::instance().drainThreadPool();
   ASSERT_EQ(EventImpl->getCommand(), nullptr);
 }
 
@@ -345,6 +349,9 @@ TEST_F(SchedulerTest, StreamBufferDeallocation) {
   // The buffers should have been released with graph cleanup once the work is
   // finished.
   EventImplPtr->wait(EventImplPtr);
+  // Drain the thread pool to ensure that the cleanup is able to acquire
+  // the graph lock.
+  detail::GlobalHandler::instance().drainThreadPool();
   MSPtr->cleanupCommands({});
   ASSERT_EQ(MSPtr->MDeferredMemObjRelease.size(), 0u);
 }