t5541: remove lockfile creation

t5541: remove lockfile creation

To create error conditions, some tests set up reference locks by
directly creating its lockfile. While this works for the files reference
backend, this approach is incompatible with the reftable backend.
Refactor the test to create a d/f conflict via git-update-ref(1) instead
so that the test is reference backend agnostic.

Signed-off-by: Justin Tobler <jltobler@gmail.com>
Acked-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>