pretty format %(trailers) test: split a long line

Split a very long line in a test introduced in 0b691d8685 (pretty:
add support for separator option in %(trailers), 2019-01-28). This
makes it easier to read, especially as follow-up commits will copy
this test as a template.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t4205-log-pretty-formats.sh

@@ -717,7 +717,12 @@ test_expect_success '%(trailers:key=foo,valueonly) shows only value' '
 
 test_expect_success 'pretty format %(trailers:separator) changes separator' '
 	git log --no-walk --pretty=format:"X%(trailers:separator=%x00,unfold)X" >actual &&
-	printf "XSigned-off-by: A U Thor <author@example.com>\0Acked-by: A U Thor <author@example.com>\0[ v2 updated patch description ]\0Signed-off-by: A U Thor <author@example.com>X" >expect &&
+	(
+		printf "XSigned-off-by: A U Thor <author@example.com>\0" &&
+		printf "Acked-by: A U Thor <author@example.com>\0" &&
+		printf "[ v2 updated patch description ]\0" &&
+		printf "Signed-off-by: A U Thor <author@example.com>X"
+	) >expect &&
 	test_cmp expect actual
 '