selftests/bpf: Add test cases for btf__permute functionality

selftests/bpf: Add test cases for btf__permute functionality

This patch introduces test cases for the btf__permute function to ensure
it works correctly with both base BTF and split BTF scenarios.

The test suite includes:
- test_permute_base: Validates permutation on base BTF
- test_permute_split: Tests permutation on split BTF

Signed-off-by: Donglin Peng <pengdonglin@xiaomi.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/bpf/20260109130003.3313716-3-dolinux.peng@gmail.com