Fix failure to copy data files if content changes

Fix failure to copy data files if content changes

Jenkins was failing to detect the case where an existing
file is recreated with new content. In this case, thinking
that the file already existed, Jenkins did not re-copy the
file as it should have.

By adding the file test-data.sha1 as a dependendency to
the LIBVPX_TEST_DATA build target the files will be
recopied if the MD5 of an existing file changes.

This could be further improved to only copy files that
have changed rather than copying the whole set as done in
this patch.

(Thanks to jzern@ who diagnozed ithe problem and suggested
this fix).

Change-Id: Icea7c61a95189bc639fec83020c28c70da5b2b41