t7519: avoid file to index mtime race for untracked cache

In t7519 there is a test that writes files to disk, and immediately writes the index with the untracked cache. Because of mtime-comparison logic that uses a 1-second resolution, this means the cached entries are not trusted/used under some circumstances (see read-cache.c#is_racy_stat()). Untracked cache tests in t7063 use a 1-second delay to avoid this issue, but we don't want to introduce arbitrary slowdowns, so instead use test-tool chmtime to backdate the files slightly. The t7063 delays are a #leftoverbit, to be worked on in a separate series. This change doesn't actually affect the outcome of the test, but does enhance its validity, and becomes relevant after later changes. Signed-off-by: Tao Klerks <tao@klerks.biz> Signed-off-by: Junio C Hamano <gitster@pobox.com>