It can be checked with Zabbix agent, system.run key.
Steps to reproduce:
1) Enable key in zabbix_agent.conf
AllowKey=system.run[*]
2) On your local machine add script external_check.sh with code

sleep 10 && echo ok

We need it to simulate timeout.
3) Create item with parameters:
Type - Zabbix agent
Key - system.run[/path/to/script/s/external_check.sh]
Type of information - Text.
4) Navigate to page Administration->General->Other
and change "Network timeout for item test" to 1s (or 2s).
5) also change "Timeout" parameter in zabbix_agent.conf to 15s
6) Navigate back to created item and try to test it.

Result:
Item testing completed (no timeout, testing not stopped after 1s as provided in configuration).
"Network timeout for item test" - this parameter ignored.

Expected:
Timeout should appear after time provided in "Network timeout for item test".