Steps to reproduce:

1. Create a simple Zabbix agent template item such as net.tcp.service[ftp,,21]
2. Click the Test button
3. Enter the host address and (in this case) port 21
4. Get value and test

Result:
Test will fail. A packet capture run on either the server/proxy conducting the test or on the host being tested will show this FTP request:
`checks","data":[{"key":"net.tcp.service[ftp,,21]","timeout":5}]}`

The FTP server will unsurprisingly return a syntax error and give up on the connection.
Expected:
The test will be sent to the zabbix agent on the indicated host just as it will be once it's a regular monitoring item and will succeed or fail on the merits of whether the tested item is healthy.