selftests/bpf: Add tests for bpf_dynptr_copy

selftests/bpf: Add tests for bpf_dynptr_copy

Add XDP setup type for dynptr tests, enabling testing for
non-contiguous buffer.
Add 2 tests:
 - test_dynptr_copy - verify correctness for the fast (contiguous
 buffer) code path.
 - test_dynptr_copy_xdp - verifies code paths that handle
 non-contiguous buffer.

Signed-off-by: Mykyta Yatsenko <yatsenko@meta.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20250226183201.332713-4-mykyta.yatsenko5@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>