Fix multi-asic behaviour for pg-drop

[bktsim-arista]

What I did

show priority-group drop is not behaving correctly on multi-asic devices, as the namespace option '-n' is not available and correct namespaces were not traversed to retrieve drop counters. This change fixes the multi-asic behaviour of this command and adds unit tests to ensure that the functionality is correct.

This is a part of the set of changes being pushed for https://github.com/sonic-net/sonic-buildimage/issues/15148

How I did it

Added multi-asic support and relevant unit tests for pg-drop script.

How to verify it

Run unit test, or pg-drop -c show with -n