Avoid global namespace pollution

Add a main() function to all custom servers based on isctest.asyncserver
and move server startup code there.  This prevents redefining variables
from outer scope in custom server code as it evolves.

bin/tests/system/qmin/ans2/ans.py

@@ -101,7 +101,7 @@ class StaleHandler(DomainHandler):
             yield send_delegation(qctx, b_stale, "10.53.0.4")
 
 
-if __name__ == "__main__":
+def main() -> None:
     server = AsyncDnsServer()
     server.install_response_handler(QueryLogger())
     server.install_response_handler(BadHandler())
@@ -109,3 +109,7 @@ if __name__ == "__main__":
     server.install_response_handler(SlowHandler())
     server.install_response_handler(StaleHandler())
     server.run()
+
+
+if __name__ == "__main__":
+    main()

bin/tests/system/qmin/ans3/ans.py

@@ -37,10 +37,14 @@ class ZoopBoingSlowHandler(DelayedResponseHandler):
     delay = 0.4
 
 
-if __name__ == "__main__":
+def main() -> None:
     server = AsyncDnsServer()
     server.install_response_handler(QueryLogger())
     server.install_response_handler(ZoopBoingBadHandler())
     server.install_response_handler(ZoopBoingUglyHandler())
     server.install_response_handler(ZoopBoingSlowHandler())
     server.run()
+
+
+if __name__ == "__main__":
+    main()

bin/tests/system/qmin/ans4/ans.py

@@ -83,7 +83,7 @@ class IckyPtangZoopBoingSlowHandler(DelayedResponseHandler):
     delay = 0.4
 
 
-if __name__ == "__main__":
+def main() -> None:
     server = AsyncDnsServer()
     server.install_response_handler(QueryLogger())
     server.install_response_handler(StaleHandler())
@@ -91,3 +91,7 @@ if __name__ == "__main__":
     server.install_response_handler(IckyPtangZoopBoingUglyHandler())
     server.install_response_handler(IckyPtangZoopBoingSlowHandler())
     server.run()
+
+
+if __name__ == "__main__":
+    main()

bin/tests/system/upforwd/ans4/ans.py

@@ -14,7 +14,11 @@ information regarding copyright ownership.
 from isctest.asyncserver import AsyncDnsServer, IgnoreAllQueries
 
 
-if __name__ == "__main__":
+def main() -> None:
     server = AsyncDnsServer()
     server.install_response_handler(IgnoreAllQueries())
     server.run()
+
+
+if __name__ == "__main__":
+    main()