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
- test_permute_drop_base: Validates type dropping on base BTF
- test_permute_drop_split: Tests type dropping on split BTF
- test_permute_drop_dedup: Tests type dropping and deduping

Cc: Eduard Zingerman <[email protected]>
Cc: Alexei Starovoitov <[email protected]>
Cc: Andrii Nakryiko <[email protected]>
Cc: Alan Maguire <[email protected]>
Cc: Song Liu <[email protected]>
Cc: Xiaoqin Zhang <[email protected]>
Signed-off-by: Donglin Peng <[email protected]>

Author: Donglin Peng