selftests/bpf: test widening for iterating callbacks

A test case to verify that imprecise scalars widening is applied to
callback entering state, when callback call is simulated repeatedly.

Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20231121020701.26440-10-eddyz87@gmail.com


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