Change status of a test (success or failed) using SOAP API
It is not possible to change/set the status of a test using the Silk Central SOAP web-service API. You can read out a test-status (property-id "_node_properties_testdef_status"), but you can't change it.
Which Silk Central version do you use?
In Silk Central 19.0 a feature called "External Executions" was introduced. This means that Silk Central extends its ability to integrate with existing tooling ecosystems by providing functionality to consume results for automated tests without having those automated tests executed through Silk Central execution servers.
For more details on External Executions, please check the online help topic "External Executions" of the Silk Central 19.0 (or higher) help.
To interact from external with such external execution plan runs, Silk Central 19.0 introduces a whole new REST API to query information around the execution plan run itself and its assigned test runs. Furthermore, the new REST API allows uploading result files and setting the status for single test runs as well as finishing the overall execution plan run.
So with this new REST API, you can set the status of single test runs of running execution plans. This is restricted to automated tests.