FTBFS in test-suite on overlayfs

Testing conf_source_reload() with job directory
...with new job directory
...with new file in directory (direct write)
BAD: wrong value for block file, got unexpected NULL
 at tests/test_conf.c:293 (test_source_reload_job_dir).
/bin/bash: line 5: 5940 Aborted (core dumped) ${dir}$tst
FAIL: test_conf

To reproduce:
$ mk-sbuild --name overlayfs raring
$ pull-lp-source upstart
$ sbuild -A -d overlayfs upstart*.dsc