Factor out context establishment in GSS tests

Add a new helper to common.c which runs gss_init_sec_context and
gss_accept_sec_context in a loop, and use it in test programs instead
of the open-coded one-token or two-token exchanges.
Add tests for GSS IOV and AEAD interfaces

Add a new test program t_iov.c which tests various combinations of
wrapping and unwrapping using the IOV and AEAD interfaces.  Run it
with and without SPNEGO in each enctype configuration.