crypto/des: test using the public API

crypto/des: test using the public API

Several of the tests in crypto/des were using the unexported
desCipher type and other unexported functions to test the package,
leaving desCipher.Encrypt and desCipher.Decrypt only partially tested.

This CL changes the tests to use the public API, except for
TestInitialPermute and TestFinalPermute, which are testing
implementation details on purpose.

Change-Id: I0bc13cea06b79b29425412b9bf36b997871518ac
Reviewed-on: https://go-review.googlesource.com/c/go/+/520495
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Filippo Valsorda <filippo@golang.org>
Run-TryBot: Quim Muntal <quimmuntal@gmail.com>
Reviewed-by: Bryan Mills <bcmills@google.com>