PhpunitTestEngine was missing some pieces in order to work. It didn't have a getEngineConfigurationName method or an instance of ArcanistConfigurationManager to work with.

Fixes T9174

Unit test included for getEngineConfigurationName method.

Tested locally against my own repo with the following:

{
  "engines": {
    "phpunit": {
      "type": "phpunit",
      "include": "(\\.php$)",
      "exclude": "(^vendor/)"
    }
  }
}