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 <[email protected]>
Reviewed-By: Joyee Cheung <[email protected]>
Reviewed-By: Colin Ihrig <[email protected]>

Author: Trott