Add regression test for GSS-API context leak via TKEY CONTINUE

Add regression test for GSS-API context leak via TKEY CONTINUE

Send crafted SPNEGO NegTokenInit tokens that propose the krb5
mechanism without a mechToken.  This causes gss_accept_sec_context()
to return GSS_S_CONTINUE_NEEDED, which on unfixed code leaks the
GSS context handle (~520 bytes per query).

The test verifies that the server rejects the negotiation (TKEY
error != 0, no continuation token) rather than returning a CONTINUE
response (error=0 with output token).