Non-diegetic panning gain not tested in ISM

The decoder command-line option for non-diegetic panning gain in ISM with 1 object is not tested in the self-test / coverage report, see e.g. https://ivas-codec-pc.3gpp.org/ivas-codec/coverage/lib_dec/ivas_mono_dmx_renderer.c.gcov.html.

It is thus proposed to add this self-test condition.