repo: Add OSTREE_REPO_TEST_ERROR=pre-commit env var

repo: Add OSTREE_REPO_TEST_ERROR=pre-commit env var

Setting this causes commit to error out.  There are other ways we
could do this in a more sophisticated fashion, such as via SystemTap
etc.  But this has low-tech applicablity, works as non-root.

The reason I'm adding this is so that we can add test cases for
cleanup of the `tmp/staging-` directory.

Closes: #170
Approved by: jlebon