t5551: test http interaction with credential helpers

t5551: test http interaction with credential helpers

We test authentication with http, and we independently test that
credential helpers work, but we don't have any tests that cover the
two features working together. Let's add two:

  1. Make sure that a successful request asks the helper to save the
     credential. This works as expected.

  2. Make sure that a failed request asks the helper to forget the
     credential. This is marked as expect_failure, as it was recently
     regressed by 1b0d9545bb (remote-curl: fall back to basic auth if
     Negotiate fails, 2021-03-22). The symptom here is that the second
     request should prompt the user, but doesn't.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>