selftests/bpf: add selftests for new insn_array map

Tests are split in two parts.

The `bpf_insn_array_ops` test checks that the map is managed properly:

  * Incorrect instruction indexes are rejected
  * Non-sorted and non-unique indexes are rejected
  * Unfrozen maps are not accepted
  * Two programs can't use the same map
  * BPF progs can't operate the map

The `bpf_insn_array` part validates, as best as it can do it from user
space, that instructions were adjusted properly:

  * 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]>

Author: aspsk