Test coverage for GSSAPI authentication

Currently, there is no test coverage for GSSAPI authentication so any use case involving GSSAPI/kerberos might be broken or can contain memory issues. This is likely because of the GSSAPI requires working kerberos setup, that is not trivial to set up, but it shold be possible to start KDC as part of the testsuite and execute important test cases against OpenSSH or libssh servers to verify the code still works and it interoperates with various servers/kerberoses.