t5516: move plaintext-password tests from t5601 and t5516

t5516: move plaintext-password tests from t5601 and t5516

Commit 6dcbdc0d66 (remote: create fetch.credentialsInUrl config,
2022-06-06) added tests for our handling of passwords in URLs. Since the
obvious URL to be affected is git-over-http, the tests use http. However
they don't set up a test server; they just try to access
https://localhost, assuming it will fail (because the nothing is
listening there).

This causes some possible problems:

  - There might be a web server running on localhost, and we do not
    actually want to connect to that.

  - The DNS resolver, or the local firewall, might take a substantial
    amount of time (or forever, whichever comes first) to fail to
    connect, slowing down the tests cases unnecessarily.

  - Since there's no server, our tests for "allow" and "warn" still
    expect the clone/fetch/push operations to fail, even though in the
    real world we'd expect these to succeed. We scrape stderr to see
    what happened, but it's not as robust as a more realistic test.

Let's instead move these to t5551, which is all about testing http and
where we have a real server. That eliminates any issues with contacting
a strange URL, and lets the "allow" and "warn" tests confirm that the
operation actually succeeds.

It's not quite a verbatim move for a few reasons:

  - we can drop the LIBCURL dependency; it's already part of
    lib-httpd.sh

  - we'll use HTTPD_URL_USER_PASS, etc, instead of our fake URL. To
    avoid repetition, we'll add a few extra variables.

  - the "https://username:@localhost" test uses a funny URL that
    lib-httpd.sh doesn't provide. We'll similarly construct it in a
    variable. Note that we're hard-coding the lib-httpd username here,
    but t5551 already does that everywhere.

  - for the "domain:port" test, the URL provided by lib-httpd is fine,
    since our test server will always be on an exotic port. But we'll
    confirm in the test that this is so.

  - since our message-matching is done via grep, I simplified it to use
    a regex, rather than trying to massage lib-httpd's variables.
    Arguably this makes it more readable, too, while retaining the bits
    we care about: the fatal/warning distinction, the "uses plaintext"
    message, and the fact that the password was redacted.

  - we'll use the /auth/ path for the repo, which shows that we are
    indeed making use of the auth information when needed.

  - we'll also use /smart/; most of these tests could be done via /dumb/
    in t5550, but setting up pushes there requires extra effort and
    dependencies. The smart protocol is what most everyone is using
    these days anyway.

This patch is my own, but I stole the analysis and a few bits of the
commit message from a patch by Johannes Schindelin.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Taylor Blau <me@ttaylorr.com>