test_sysctl: test against int proc_dointvec() array support

Add a few initial respective tests for an array:

  o Echoing values separated by spaces works
  o Echoing only first elements will set first elements
  o Confirm PAGE_SIZE limit still applies even if an array is used

Cc: Kees Cook <keescook@chromium.org>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Luis R. Rodriguez <mcgrof@kernel.org>