bug: benchmark panic

[lucasmenendez]

Describe the bug
Panic error initializing MockVochainNode on benchmarks

To Reproduce (please complete the following information)

Go to benchmark/ folder run, for example, the following prompt:

go test -v -run=- -bench=BenchmarkVochainSingleProof

Current behavior
Panic caused by invalid memory address or nil pointer dereference trying to update the vochain configuration struct data directory (config.VochainCfg.DataDir) with the testing temporal directory (testing.TB.TempDir()):

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x70 pc=0x2ccda5d]

goroutine 23 [running]:
go.vocdoni.io/dvote/test/testcommon.NewMockVochainNode({0x3e15a50, 0xc0005c0000}, 0x0, 0x6?)
        /home/lucasmenendez/workspace/vocdoni-node/test/testcommon/vochain.go:193 +0x5d
go.vocdoni.io/dvote/test/testcommon.(*DvoteAPIServer).Start(0xc0001d3560, {0x3e15a50, 0xc0005c0000}, {0xc002a4fee8, 0x3, 0x116c65e?})
        /home/lucasmenendez/workspace/vocdoni-node/test/testcommon/rpc.go:102 +0x777
go.vocdoni.io/dvote/benchmark.BenchmarkVochainSingleProof(0xc0005c0000)
        /home/lucasmenendez/workspace/vocdoni-node/benchmark/vochain_benchmark_test.go:90 +0x119
testing.(*B).runN(0xc0005c0000, 0x1)
        /usr/local/go/src/testing/benchmark.go:193 +0x102
testing.(*B).run1.func1()
        /usr/local/go/src/testing/benchmark.go:233 +0x59
created by testing.(*B).run1
        /usr/local/go/src/testing/benchmark.go:226 +0x9c
exit status 2
FAIL    go.vocdoni.io/dvote/benchmark   0.918s

Expected behavior
I assume that the expected behaviour of this benchmark is to run successfully.

System (please complete the following information):

• OS: Debian 5.10.162-1 (2023-01-21) x86_64 GNU/Linux
• Software version: go version go1.20.3 linux/amd64
• Commit hash: c9425c1