selftests/bpf: add selftests for new insn_array map

selftests/bpf: add selftests for new insn_array map

Add the following selftests for new insn_array map:

  * Incorrect instruction indexes are rejected
  * Two programs can't use the same map
  * BPF progs can't operate the map
  * no changes to code => map is the same
  * expected changes when instructions are added
  * expected changes when instructions are deleted
  * expected changes when multiple functions are present

Signed-off-by: Anton Protopopov <a.s.protopopov@gmail.com>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20251105090410.1250500-5-a.s.protopopov@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>