Added coverage for the addnode rpc when v2transport is not enabled,
but is set as true when calling addnode rpc.
I ran the following to check if this rpc error message
was covered in the functional tests.
grep -nr "v2transport requested but not enabled" ./test/functional --binary-files=without-match
Adds test coverage to this line. https://github.com/bitcoin/bitcoin/blob/master/src/rpc/net.cpp#L339