selftests/bpf: Add tests for add_const

Improve arena based tests and add several C and asm tests
with specific pattern.
These tests would have failed without add_const verifier support.

Also add several loop_inside_iter*() tests that are not related to add_const,
but nice to have.

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20240613013815.953-5-alexei.starovoitov@gmail.com