selftests/bpf: Test bpf_sysctl_get_name helper

Test w/ and w/o BPF_F_SYSCTL_BASE_NAME, buffers with enough space and
too small buffers to get E2BIG and truncated result, etc.

  # ./test_sysctl
  ...
  Test case: sysctl_get_name sysctl_value:base ok .. [PASS]
  Test case: sysctl_get_name sysctl_value:base E2BIG truncated .. [PASS]
  Test case: sysctl_get_name sysctl:full ok .. [PASS]
  Test case: sysctl_get_name sysctl:full E2BIG truncated .. [PASS]
  Test case: sysctl_get_name sysctl:full E2BIG truncated small .. [PASS]
  Summary: 11 PASSED, 0 FAILED

Signed-off-by: Andrey Ignatov <rdna@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>