selftests/bpf: Add tests for loading insn array values with offsets

selftests/bpf: Add tests for loading insn array values with offsets

The ldimm64 instruction for map value supports an offset.
For insn array maps it wasn't tested before, as normally
such instructions aren't generated. However, this is still
possible to pass such instructions, so add a few tests to
check that correct offsets work properly and incorrect
offsets are rejected.

Signed-off-by: Anton Protopopov <a.s.protopopov@gmail.com>
Link: https://lore.kernel.org/r/20260111153047.8388-4-a.s.protopopov@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>