t6300: drop newline from wrapped test title

We don't usually include newlines in test titles, because you get funny TAP output like: ok 417 - show good signature with custom format ok 418 - show good signature with custom format with ssh ok 419 - signature atom with grade option and bad signature where a TAP parser would ignore the extra line anyway, giving the wrong title. This comes from 26c9c03f0a (ref-filter: add new "signature" atom, 2023-06-04), and I think it was probably just editor line wrapping. I checked for other cases with: git grep "test_expect_success [A-Z_,]* '[^']*$" git grep 'test_expect_success [A-Z_,]* "[^"]*$' but this was the only hit. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>