t5563: add tests for http.emptyAuth with Negotiate

t5563: add tests for http.emptyAuth with Negotiate

Add tests exercising the interaction between http.emptyAuth and
servers that advertise Negotiate (SPNEGO) authentication.

Verify that auto mode gives Negotiate a chance via empty auth
(resulting in two 401 responses before falling through to
credential_fill with Basic credentials), and that false mode
strips Negotiate immediately (only one 401 response).

Signed-off-by: Matthew John Cheetham <mjcheetham@outlook.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>