Comment 9 for bug 904014

I'm afraid I don't have the debhelper incantation OTOH to give you. With libvirt I have been known to patch a test case to output that it failed, but return 0. Eg, from 9008-fix-daemon-conf-ftbfs.patch in lucid's libvirt:
-wait $pid || fail=1
+#wait $pid || fail=1
+# warn, but do not fail since Ubuntu buildds do not work too well with this test
+wait $pid || {
+ printf "WARNING: wait failed (TODO: update test suite). Reason:\n" 1>&2
+ cat log 1>&2
+}

You could apply a similar methodology.