Before, a global -v2transport provided to the test would be dropped
when restarting the node within a test and specifying any extra_args.
Fix this by adding "v2transport=1" to args (not extra_args) based
on the global parameter, and deciding for each (re)start of the node
based on this default and test-specific extra_args
(which take precedence over args) whether v2 should be used.