t/lib-chunk: work around broken "mv" on some vintage of macOS
When the destination is read-only, "mv" on some version of macOS
asks whether to replace the destination even though in the test its
stdin is not a terminal (and thus doesn't conform to POSIX[1]).
The helper to corrupt a chunk-file is designed to work on the
files like commit-graph and multi-pack-index files that are
generally read-only, so use "mv -f" to work around this issue.
Helped-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>