t5551: simplify expected cookie file

After making an HTTP request that should store cookies, we check that the expected values are in the cookie file. We don't want to look at the whole file, because it has noisy comments at the top that we shouldn't depend on. But we strip out the interesting bits using "tail -3", which is brittle. It requires us to put an extra blank line in our expected output, and it would fail to notice any reordering or extra content in the cookie file. Instead, let's just grep for non-blank lines that are not comments, which more directly describes what we're interested in. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>