selftests/bpf: check that modifier resolves after pointer

Add a regression test that ensures that a VAR pointing at a
modifier which follows a PTR (or STRUCT or ARRAY) is resolved
correctly by the datasec validator.

Signed-off-by: Lorenz Bauer <lmb@isovalent.com>
Link: https://lore.kernel.org/r/20230306112138.155352-3-lmb@isovalent.com


Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>