Previously this test suite used a short-cut that didn't work well with the new crypto
API being experimented with in other branches.
This commit uses the legacy decode and decryption functions in ssh_transport. It is of
course normally a bad idea to test something with itself. However, other test suites
test the interoperability with OpenSSH and by that implicitly of the decoding/decryption.
The ssh_protocol_SUITE tests sending malformed and even malicious PDUs to the Erlang/OTP
SSH implementation. Therefore the tests in this suite is not dependent on being different
than thoose of the system under test.