tests: net: mgmt: define request handler properly

Define the request handler properly within the net mgmt test. This is
required for the alias() attribute to work correctly with clang. Also,
correct the handler signature to use size_t as expected. This resolves a
test failure seen with clang when building the net mgmt test for
native_sim.

Signed-off-by: Daniel DeGrasse <ddegrasse@tenstorrent.com>