selftests/bpf: Add selftest for bits iter

selftests/bpf: Add selftest for bits iter

Add test cases for the bits iter:

- Positive cases
  - Bit mask representing a single word (8-byte unit)
  - Bit mask representing data spanning more than one word
  - The index of the set bit

- Nagative cases
  - bpf_iter_bits_destroy() is required after calling
    bpf_iter_bits_new()
  - bpf_iter_bits_destroy() can only destroy an initialized iter
  - bpf_iter_bits_next() must use an initialized iter
  - Bit mask representing zero words
  - Bit mask representing fewer words than expected
  - Case for ENOMEM
  - Case for NULL pointer

Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20240517023034.48138-3-laoar.shao@gmail.com