selftests/bpf: Add tests for maybe_fork_scalars() OR vs AND handling

Add three test cases to verifier_bounds.c to verify that
maybe_fork_scalars() correctly tracks register values for BPF_OR
operations with constant source operands:

1. or_scalar_fork_rejects_oob: After ARSH 63 + OR 8, the pushed
   path should have dst = 8. With value_size = 8, accessing
   map_value + 8 is out of bounds and must be rejected.

2. and_scalar_fork_still_works: Regression test ensuring AND
   forking continues to work. ARSH 63 + AND 4 produces pushed
   dst = 0 and current dst = 4, both within value_size = 8.

3. or_scalar_fork_allows_inbounds: After ARSH 63 + OR 4, the
   pushed path has dst = 4, which is within value_size = 8
   and should be accepted.

These tests exercise the fix in the previous patch, which makes the
pushed path re-execute the ALU instruction so it computes the correct
result for BPF_OR.

Signed-off-by: Daniel Wade <danjwade95@gmail.com>
Reviewed-by: Amery Hung <ameryhung@gmail.com>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20260314021521.128361-3-danjwade95@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>