test: Remove UninterruptibleSleep from test and replace it by SyncWithValidationInterfaceQueue

For the purpose of this test the two have the same outcome, but this one
is shorter and avoids a sleep for 0.1 seconds.

src/test/validation_block_tests.cpp

@@ -204,9 +204,7 @@ BOOST_AUTO_TEST_CASE(processnewblock_signals_ordering)
     for (auto& t : threads) {
         t.join();
     }
-    while (GetMainSignals().CallbacksPending() > 0) {
-        UninterruptibleSleep(std::chrono::milliseconds{100});
-    }
+    SyncWithValidationInterfaceQueue();
 
     UnregisterValidationInterface(&sub);