Add another workaround for failures in wxDateTime::UNow test

If current time doesn't have the milliseconds parts, we need to sleep
for a little before retrying.

tests/datetime/datetimetest.cpp

@@ -1743,6 +1743,8 @@ TEST_CASE("wxDateTime::UNow", "[datetime][now][unow]")
             gotMS = true;
             break;
         }
+
+        wxMilliSleep(123);
     }
 
     CHECK( gotMS );