t/helper/test-write-cache: clean up lock-handling

t/helper/test-write-cache: clean up lock-handling

Die in case writing the index fails, so that the caller can notice
(instead of, say, being impressed by how performant the writing is).

While at it, note that after opening a lock with `LOCK_DIE_ON_ERROR`, we
do not need to worry about whether we succeeded. Also, we can move the
`struct lock_file` into the function and drop the staticness. (Placing
`struct lock_file`s on the stack used to be a bad idea, because the
temp- and lockfile-machinery would keep a pointer into the struct. But
after 076aa2cbd (tempfile: auto-allocate tempfiles on heap, 2017-09-05),
we can safely have lockfiles on the stack.)

Signed-off-by: Martin Ågren <martin.agren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>