Increase deadlock_timeout to 100ms in FK isolation tests

Increase deadlock_timeout to 100ms in FK isolation tests

The previous value of 20ms is dangerously close to the time actually
spent just waiting for the deadlock to happen, so on occasion it causes
the test to fail simply because the other session didn't get to run
early enough, not managing to cause the deadlock that needs to be
detected.  With this new value, it's expected that most machines on
normal load will be able to pass the test.

Author: Noah Misch