t5319: replace 'touch -m' with 'test-tool chmtime'

t5319: replace 'touch -m' with 'test-tool chmtime'

The use of 'touch -m' to modify a file's mtime is slightly less
portable than using our own 'test-tool chmtime'. The important
thing is that these pack-files are ordered in a special way to
ensure the multi-pack-index selects some as the "newer" pack-files
when resolving duplicate objects.

Reported-by: Jeff King <peff@peff.net>
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>