selftests/bpf: Add tests for reference state fixes for callbacks

These are regression tests to ensure we don't end up in invalid runtime
state for helpers that execute callbacks multiple times. It exercises
the fixes to verifier callback handling for reference state in previous
patches.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20220823013226.24988-1-memxor@gmail.com


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