Add selftest for libbpf functions libbpf_prog_type_by_name and
libbpf_attach_type_by_name.

Example of output:
  % ./tools/testing/selftests/bpf/test_section_names
  Summary: 35 PASSED, 0 FAILED
Signed-off-by: Andrey Ignatov <rdna@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>