t: add a test helper to truncate files

t: add a test helper to truncate files

In a future commit, we're going to work with some large files which will
be at least 4 GiB in size.  To take advantage of the sparseness
functionality on most Unix systems and avoid running the system out of
disk, it would be convenient to use truncate(2) to simply create a
sparse file of sufficient size.

However, the GNU truncate(1) utility isn't portable, so let's write a
tiny test helper that does the work for us.

Signed-off-by: brian m. carlson <bk2204@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>