test: decrease duration of test-cli-syntax

Previously, test/parallel/test-cli-syntax.js was spawning a lot of child
processes, but using spawnSync, which made the test run each child
process serially. This switches most of the test cases to use exec so
that they are asynchronous. Locally, the test went from > 5 seconds to
under 2 seconds.

PR-URL: https://github.com/nodejs/node/pull/14187


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>