t5551: handle trailing slashes in expected cookies output

We check in t5551 that curl updates the expected list of cookies after making a request. We do this by telling it to read and write cookies from a particular text file, and then checking that after curl runs, the file has the expected content. However, in the upcoming curl 8.18.0, the output file has changed slightly: curl will canonicalize the paths it writes, due to commit a093c93994 (cookie: only keep and use the canonical cleaned up path, 2025-12-07). In particular, it strips trailing slashes from the paths we see in the cookies.txt file. This doesn't matter to Git, as the cookie handling is all internal to curl. But our test is overly brittle and breaks as a result. We can fix it by matching either format. We'll expect the new format (without trailing slashes) and strip the slashes from curl's output before comparing. That lets us pass with both old and new versions (I tested against curl's 8_17_0 and rc-8_18_0-2 tags, which are respectively before and after the curl change). In theory it might be nice to try to future-proof this test more by looking only for the bits we care about, rather than a byte-wise comparison of the whole file. But after removing comments and blank lines (which we already do), we care about most of what's there. So it's not clear to me what a more liberal test would look like. Given that the format doesn't change all that often, it's probably OK to stop here and see if it ever breaks again. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>