selftests/bpf: test_xsk: Wrap test clean-up in functions

selftests/bpf: test_xsk: Wrap test clean-up in functions

The clean-up done at the end of a test in __testapp_validate_traffic()
isn't wrapped in a function. It isn't convenient if we want to use it
somewhere else in the code.

Wrap the clean-up in two new functions : the first deletes the sockets,
the second releases the umem.

Reviewed-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Signed-off-by: Bastien Curutchet (eBPF Foundation) <bastien.curutchet@bootlin.com>
Link: https://lore.kernel.org/r/20251031-xsk-v7-6-39fe486593a3@bootlin.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>