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.

(cherry picked from commit 8cb51d4c2b)

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()