Currently Spectrum only supports the SPM Test Discovery for Cisco devices, and SystemEDG_Hosts. The Juniper devices are not supported for test discovery.
The "Spectrum Performance Manager" guide - "Working with Performance Tests" - "Discover Tests in the Network" section, contains the requirements for running SPM Test Discovery, which states:
• Administrator role privileges are required to use SPM Test Discovery.
• Use a device that SPM Test Discovery supports."
See the Spectrum Performance Manager Guide - Discover Tests in the Network for more information
This feature is not supported for all RTM_TestHosts, but rather for specific devices.
For a list of what device/agents are supported by Spectrum Performance Manager, and what features and tests that each agent supports see the "Spectrum Performance Manager" guide - "Performance Agents and MIBs" section. The only two Agent/MIB that are listed, which Spectrum supports SPM Test Discovery for are the Cisco IOS SLAs Agent, and the CA eHealth SystemEDGE agent.