test: move test-cli-syntax to sequential

It is unreliable under load and the CI failures are getting a bit out of
hand. Let's move it to sequential.

Refs: #24403

PR-URL: #24907
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

Author: Trott