lint: fix integration test

The body of an ATF test must never return 1 but instead report failure
via atf_fail.  Otherwise the following error message appears:

    Failed: Test case body returned a non-ok exit code, but this is
    not allowed

The test program t_integration intentionally bypasses the official ATF
API for performance reasons.  But even then, it should stick to the API
as close as possible.

(rillig)