tests: socket: udp: reduce SO_PRIORITY from 8 to 6 for NSOS compatbility

According to 'man 7 socket' about SO_PRIORITY:

  Setting a priority outside the range 0 to 6 requires the CAP_NET_ADMIN
  capability.

So use 6 instead of 8, in order to make UDP socket tests pass with native
offloaded sockets on native_sim platform.

Signed-off-by: Marcin Niestroj <m.niestroj@emb.dev>