[network] Default ipv6 to false to always set the flags (#6937)

* [network] Default ipv6 to false to always set the flags

* Separate tests for ipv6 disabled and enabled

* Forgot other platforms wouldnt have the variable in config

tests/components/network/common.yaml

@@ -1,6 +1,9 @@
+substitutions:
+  network_enable_ipv6: "false"
+
 wifi:
   ssid: MySSID
   password: password1
 
 network:
-  enable_ipv6: true
+  enable_ipv6: ${network_enable_ipv6}