selftests/xsk: Add tail adjustment tests and support check

Introduce tail adjustment functionality in xskxceiver using
bpf_xdp_adjust_tail(). Add `xsk_xdp_adjust_tail` to modify packet sizes
and drop unmodified packets. Implement `is_adjust_tail_supported` to check
helper availability. Develop packet resizing tests, including shrinking
and growing scenarios, with functions for both single-buffer and
multi-buffer cases. Update the test framework to handle various scenarios
and adjust MTU settings. These changes enhance the testing of packet tail
adjustments, improving AF_XDP framework reliability.

Reviewed-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Signed-off-by: Tushar Vyavahare <tushar.vyavahare@intel.com>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Link: https://patch.msgid.link/20250410033116.173617-3-tushar.vyavahare@intel.com