selftests: net: tsn_lib: create common helper for counting received packets

This snippet will be necessary for a future isochron-based test, so
provide a simpler high-level interface for counting the received
packets.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Link: https://patch.msgid.link/20250426144859.3128352-3-vladimir.oltean@nxp.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>