selftests/bpf: Add test for add_const base_id consistency

selftests/bpf: Add test for add_const base_id consistency

Add a test to verifier_linked_scalars that exercises the base_id
consistency check for BPF_ADD_CONST linked scalars during state
pruning.

With the fix, pruning fails and the verifier discovers the true
branch's R3 is too wide for the stack access.

  # LDLIBS=-static PKG_CONFIG='pkg-config --static' ./vmtest.sh -- ./test_progs -t verifier_linked_scalars
  [...]
  #613/22  verifier_linked_scalars/scalars_stale_delta_from_cleared_id:OK
  #613/23  verifier_linked_scalars/scalars_stale_delta_from_cleared_id_alu32:OK
  #613/24  verifier_linked_scalars/linked scalars: add_const base_id must be consistent for pruning:OK
  #613     verifier_linked_scalars:OK
  Summary: 1/24 PASSED, 0 SKIPPED, 0 FAILED

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/r/20260410232651.559778-2-daniel@iogearbox.net
Signed-off-by: Alexei Starovoitov <ast@kernel.org>