selftests/bpf: Test cases for missing spill types

A few selftests checking that the verifier represents spills for the
following pointer types w/o losing precision:
- PTR_TO_INSN
- PTR_TO_TP_BUFFER
- CONST_PTR_TO_DYNPTR

Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/bpf/20260707-missing-spillable-types-v1-2-44a92121dc41@gmail.com
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>