test: slightly refactor test-child-process-execsync

* move `start` time to the point of execution (avoids counting 'throws'
  tests towards 'timeout' test case)
* scope cmd/ret values where possible
* use `filter` instead of manual if/return

PR-URL: https://github.com/nodejs/node/pull/25227
Refs: https://github.com/nodejs/node/issues/24921


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>