The 'testfile' environment variable is initialized before the
xfstests environment is included into generic/313. TEST_DIR is not
defined at this point and causes the test to operate on the root.
Move the testfile initialization down after the general environment
is sourced.
Signed-off-by: Brian Foster <bfoster@redhat.com> Reviewed-by: Jie Liu <jeff.liu@oracle.com> Signed-off-by: Dave Chinner <david@fromorbit.com>