test: fix flaky message pool integration tests

Using the same pattern described in my previous commit.
I also added the CircuitBreaker to the itests kit as it may be useful
for other integration tests when debugging flakyness caused by timeouts.

itests/kit/circuit.go

@@ -0,0 +1,34 @@
+package kit
+
+import (
+	"fmt"
+	"testing"
+	"time"
+)
+
+/*
+CircuitBreaker implements a simple time-based circuit breaker used for waiting for async operations to finish.
+
+This is how it works:
+	- It runs the `cb` function until it returns true,
+	- waiting for `throttle` duration between each iteration,
+	- or at most `timeout` duration until it breaks test execution.
+
+You can use it if t.Deadline() is not "granular" enough, and you want to know which specific piece of code timed out,
+or you need to set different deadlines in the same test.
+*/
+func CircuitBreaker(t *testing.T, label string, throttle, timeout time.Duration, cb func() bool) {
+	tmo := time.After(timeout)
+	for {
+		if cb() {
+			break
+		}
+		select {
+		case <-tmo:
+			t.Fatal("timeout: ", label)
+		default:
+			fmt.Printf("waiting: %s\n", label)
+			time.Sleep(throttle)
+		}
+	}
+}