MSVC: Fix signed/unsigned mismatch errors

  test_digest.c(513,2): warning C4389: '==': signed/unsigned mismatch
  test_digest.c(820,5): warning C4389: '==': signed/unsigned mismatch
  test_sign.c(241,22): warning C4389: '==': signed/unsigned mismatch
  test_params.c(1131,16): warning C4018: '<': signed/unsigned mismatch
  test_sign.c(241,22): warning C4389: '==': signed/unsigned mismatch

Signed-off-by: Vitaly Chikunov <vt@altlinux.org>