[TEST] Reduce stringency of breaker assertions

While in a perfect world we should only ever have 2 circuit breaker
trips, it's possible to get a race condition between the child and the
parent breaker with many threads. Since multiple breaking exceptions are
not actually a bad thing, it's okay to relax the constraints in the
test.

The race conditions are due to no locking inside the breaker logic, to
ensure that it is as low overhead as possible. Even though no locking is
used, we use atomic counters internally to ensure that the "estimated"
numbers for the breakers are never out of sync (which this test still
checks with no leeway).