This subtemplate simply echoes all the parameters it was called with, surrounding these within square brackets. The template itself is not a test of Module:Params, but it is designed to be used as a callback by other test templates. As such, it should be kept immutable.
Usage
{{Module:Params/testcases/tdummy echo sb|foo|bar|Jupiter|hello=world}}- ↳ [1=foo][2=bar][3=Jupiter][hello=world]