Previously it was only possible to request them with the exact template name, e.g. 'src_%s', but not with "instantiated" names that would match this template, e.g.'src_foo_bar'. This is now possible and a test was added for this, in addition to fixing a previously invalid test. Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/2635>