Test namepace at the same/lower level than prod one not found

[Idrinth]

De\Idrinth\TestGenerator\AClass => De\Idrinth\Test\TestGenerator\AClass
Given

{
  "autoload": {
    "psr-4": {
      "De\\Idrinth\\TestGenerator\\": "src"
    }
  },
  "autoload-dev": {
    "psr-4": {
      "De\\Idrinth\\Test\\": "test"
    }
  }
}