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>