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 <[email protected]>
Acked-by: Eduard Zingerman <[email protected]>

Author: aspsk