test: refactor common.ddCommand()

* Remove different paths for Windows and POSIX.
* Remove fixtures file. Simply run the command immediately/directly.
* Since it is never called with more than one value for kilobytes,
  eliminate that argument.
* Update/simplify tests that use this function. (They no longer need to
  use child_process to run the command.)
* Update documentation.

PR-URL: https://github.com/nodejs/node/pull/23411


Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: George Adams <george.adams@uk.ibm.com>