selftests/bpf: add verifier cgroup storage tests

Add the following verifier tests to cover the cgroup storage
functionality:
1) valid access to the cgroup storage
2) invalid access: use regular hashmap instead of cgroup storage map
3) invalid access: use invalid map fd
4) invalid access: try access memory after the cgroup storage
5) invalid access: try access memory before the cgroup storage
6) invalid access: call get_local_storage() with non-zero flags

For tests 2)-6) check returned error strings.

Expected output:
  $ ./test_verifier
  #0/u add+sub+mul OK
  #0/p add+sub+mul OK
  #1/u DIV32 by 0, zero check 1 OK
  ...
  #280/p valid cgroup storage access OK
  #281/p invalid cgroup storage access 1 OK
  #282/p invalid cgroup storage access 2 OK
  #283/p invalid per-cgroup storage access 3 OK
  #284/p invalid cgroup storage access 4 OK
  #285/p invalid cgroup storage access 5 OK
  ...
  #649/p pass modified ctx pointer to helper, 2 OK
  #650/p pass modified ctx pointer to helper, 3 OK
  Summary: 901 PASSED, 0 SKIPPED, 0 FAILED

Signed-off-by: Roman Gushchin <guro@fb.com>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>