selftests/bpf: Add unit tests for new bswap insns

Add unit tests for bswap insns.

Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20230728011314.3720109-1-yonghong.song@linux.dev


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