Reenable multithread scheduler test

Github-Pull: #8016
Rebased-From: db18ab28c7a74bb289bfe6a5f9a4a9f963f71c0b 166e4b0dfa283fbdedc9a6a1e83296500c853a31

src/scheduler.cpp

@@ -79,6 +79,7 @@ void CScheduler::serviceQueue()
         }
     }
     --nThreadsServicingQueue;
+    newTaskScheduled.notify_one();
 }
 
 void CScheduler::stop(bool drain)

src/test/scheduler_tests.cpp

@@ -40,7 +40,6 @@ static void MicroSleep(uint64_t n)
 #endif
 }
 
-#if 0 /* Disabled for now because there is a race condition issue in this test - see #6540 */
 BOOST_AUTO_TEST_CASE(manythreads)
 {
     seed_insecure_rand(false);
@@ -116,6 +115,5 @@ BOOST_AUTO_TEST_CASE(manythreads)
     }
     BOOST_CHECK_EQUAL(counterSum, 200);
 }
-#endif
 
 BOOST_AUTO_TEST_SUITE_END()