The `nut-driver-enumerator.sh` script (NDE) would benefit from a CI mock service backend

[jimklimov]

The NDE script is written in an extensible fashion, as much as shell permits that, and can tap into systemd or SMF service management frameworks to define nut-driver unit instances with their custom dependency configurations, possibly MD5-ized names, config content checksums, etc.

Using real frameworks may be unwieldy for NDE testing and non-regression validation (in CI or during development), so a "mock framework" which would store the information and make-believe about service stop/start activity would be useful.

Inspired by work on #2410 which may be a fallout of #682 change set.

NOTE: The source code does already include a "selftest" frameworks support, with all hook methods currently leading to a selftest_NOOP() implementation.