t3306,t5304: avoid clock skew issues

t3306,t5304: avoid clock skew issues

On systems where the local time and file modification time may be out of
sync (e.g. test directory on NFS) t3306 and t5305 can fail because prune
compares times such as "now" (client time) with file modification times
(server times for remote file systems). I.e., these are spurious test
failures.

Avoid this by setting the relevant modification times to the local time.

Noticed on a system with as little as 2s time skew.

Signed-off-by: Michael J Gruber <git@drmicha.warpmail.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>