Steps to reproduce:

It is possible to successfully complete the testing of an item using offline proxy / non-existing proxy on the frontend. No data about item test is appearing in the proxy log in case of testing via online proxy.
 
Precondition: create proxy

1. Start server, agent and proxy
2. Open Data collection -> Hosts -> Zabbix server
3. Open any item that has Test option (for example: Linux available memory)
4. Open Test window
5. Select proxy for testing
6. Click Get Value and test
7. Review the result
8. Turn off the proxy
9. Repeat same steps with testing same item
10. Review the results
11. Check the proxy log

Result:
Results on the frontend are not affected by proxy status. 
Data about tested item  is not present in the proxy log file.

Log file with testing Linux available memory item using passive proxy (increased trapper debug level): proxy-trapper-log.txt

See video (testing item with offline proxy):

Expected:
In cases when proxy is offline - testing by proxy should not be possible.
Testing via online proxy should work correctly.