TOCTOU Bug in Circuit Breaker

In each test we create circuit breaker:

func New(name string, config Config) *CircuitBreaker {
	breaker := manager.GetCircuit(name)
	if breaker != nil {
		return &CircuitBreaker{
			Circuit: breaker,
		}
	}
	/* ... */
	breaker = manager.MustCreateCircuit(name, /* ... */)
}

Here we have a TOCTOU bug, which sometimes causes test to flap:

--- FAIL: TestColdStoreIntegration (21.99s)
    --- FAIL: TestColdStoreIntegration/TestAgg (0.01s)
        manager.go:69: test panicked: circuit with that name already exists

Start at manager.go:69 and trace the New function and the manager circuit lookup and creation calls shown in the issue. Run TestColdStoreIntegration/TestAgg to reproduce the panic, then verify that concurrent circuit creation no longer reports that the circuit already exists and the test stops flapping.