rpc/server: use dynamic port for P2P service
Fix running tests alongside regular testnet node.
panic: TCP listen error
goroutine 4634 [running]:
go.uber.org/zap/zapcore.(*CheckedEntry).Write(0xc0008540c0, 0xc001a4df80, 0x1, 0x1)
/home/rik/go/pkg/mod/go.uber.org/zap@v1.18.1/zapcore/entry.go:234 +0x532
go.uber.org/zap.(*Logger).Panic(0xc002399ce0, 0xecd669, 0x10, 0xc001a4df80, 0x1, 0x1)
/home/rik/go/pkg/mod/go.uber.org/zap@v1.18.1/logger.go:230 +0x85
github.com/nspcc-dev/neo-go/pkg/network.(*TCPTransport).Accept(0xc000fd6960)
/home/rik/dev/neo-go/pkg/network/tcp_transport.go:49 +0x216
created by github.com/nspcc-dev/neo-go/pkg/network.(*Server).Start
/home/rik/dev/neo-go/pkg/network/server.go:292 +0x2bb
FAIL github.com/nspcc-dev/neo-go/pkg/rpc/server 1.095s