t6300: drop newline from wrapped test title

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>