test: use absolute import in test_net

use absolute to avoid import twice network_build, which will call twice
exec(fixnemu)

re6st/tests/test_network/test_net.py

@@ -7,8 +7,7 @@ import logging
 import random
 from pathlib2 import Path
 
-import network_build
-import re6st_wrap
+from re6st.tests.test_network import my_net, network_build, re6st_wrap
 
 PING_PATH = str(Path(__file__).parent.resolve() / "ping.py")