t5550: factor out http auth setup

t5550: factor out http auth setup

The t5550 script sets up a nice askpass helper for
simulating user input and checking what git prompted for.
Let's make it available to other http scripts by migrating
it to lib-httpd.

We can use this immediately in t5540 to make our tests more
robust (previously, we did not check at all that hitting the
password-protected repo actually involved a password).
Unfortunately, we end up failing the test because the
current code erroneously prompts twice (once for
git-remote-http, and then again when the former spawns
git-http-push).

More importantly, though, it will let us easily add
smart-http authentication tests in t5541 and t5551; we
currently do not test smart-http authentication at all.

As part of making it generic, let's always look for and
store auxiliary askpass files at the top-level trash
directory; this makes it compatible with t5540, which runs
some tests from sub-repositories. We can abstract away the
ugliness with a short helper function.

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