selftests/bpf: test_xsk: Integrate test_xsk.c to test_progs framework

selftests/bpf: test_xsk: Integrate test_xsk.c to test_progs framework

test_xsk.c isn't part of the test_progs framework.

Integrate the tests defined by test_xsk.c into the test_progs framework
through a new file : prog_tests/xsk.c. ZeroCopy mode isn't tested in it
as veth peers don't support it.

Move test_xsk{.c/.h} to prog_tests/.

Add the find_bit library to test_progs sources in the Makefile as it is
is used by test_xsk.c

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-v7-15-39fe486593a3@bootlin.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>