In some cases, the numeric identifier doesn't correspond to the
directory name (i.e. `resolver` server in `shutdown` test, which is
supposed to be 10.53.0.3). These are typically servers that shouldn't be
auto-started by the runner, thus avoiding the typical `*ns<X>` name.
Support these server by allowing a fallback initialization with custom
numeric identifier in case it can't be parsed from the directory name.
a24f71bae4