tests: net: socket: af_packet: Simplify test initialization

Test interface configuration doesn't change throught the test suite
execution, therefore there's no need to reiterate the interfaces to set
the interface pointers in each test case. Do it just once, when test
suite is setup.

Also remove the __test_packet_sockets() function which seemed no longer
useful with all those simplifications.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>