fix and cleanup des and 3des test cases

 - Drop unused 'num' field from struct des_test_case.
 - Fix the order of arguments passed to compare_testvector() (actual
   and expected buffers were swapped, leading to misleading error
   messages for failing tests).
 - Enable all DES test vectors by default and use them for both
   encrypt and decrypt. That allows the struct des_test_case 'mode'
   field (which was previously incorrect for the LTC_TEST_EXT tests)
   to be dropped.
 - Run the "encrypt / decrypt all zero's" tests once, instead of
   running repeatedly from within the test vectors loop.
 - Add minimal set of 128bit key 3DES test vectors.
 - Try to more closely align the des_test() and des3_test() functions
   (common flow, common variable names, etc).
 - Minor indent fixes.

Signed-off-by: Andre McCurdy <armccurdy@gmail.com>

Author: armcc