t/64_nullable_foreign_key_handling_on_insert_and_delete.t fails randomly

[brummett]

We've seen this infrequently in internal testing, but it also fails from time to time on CPAN testers:
http://www.cpantesters.org/cpan/report/c166ac28-8649-11e6-ab41-c893a58a4b8c

[brummett]

A few more from the most recent release:
http://www.cpantesters.org/cpan/report/ab048eb2-83c6-11e6-ab41-c893a58a4b8c
http://www.cpantesters.org/cpan/report/65efa9f2-822b-11e6-9919-d235d7926715
http://www.cpantesters.org/cpan/report/2da98a46-8180-11e6-ab41-c893a58a4b8c