bpf: selftests: update xdp_adjust_tail selftest to include xdp frags

This change adds test cases for the xdp frags scenarios when shrinking
and growing.

Acked-by: Toke Hoiland-Jorgensen <toke@redhat.com>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Co-developed-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Eelco Chaudron <echaudro@redhat.com>
Link: https://lore.kernel.org/r/d2e6a0ebc52db6f89e62b9befe045032e5e0a5fe.1642758637.git.lorenzo@kernel.org


Signed-off-by: Alexei Starovoitov <ast@kernel.org>