Reenable multithread scheduler test.

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()