[TESTS] Be less strict about breaker child limit

Failing a parent breaker check is eventually consistent, so the test
could fail the parent limit, throw an exception, and before being
adjusted back down, increment more and throw a circuit breaking
exception on the child. This increases the child's limit, to ensure
we're only testing the parent limit.

It adds an additional assert to ensure that the breaker total is
correctly re-adjusted when the parent breaker has been tripped.

src/test/java/org/elasticsearch/common/breaker/MemoryCircuitBreakerTests.java

@@ -144,6 +144,7 @@ public class MemoryCircuitBreakerTests extends ElasticsearchTestCase {
         final int NUM_THREADS = scaledRandomIntBetween(3, 15);
         final int BYTES_PER_THREAD = scaledRandomIntBetween(500, 4500);
         final int parentLimit = (BYTES_PER_THREAD * NUM_THREADS) - 2;
+        final int childLimit = parentLimit + 10;
         final Thread[] threads = new Thread[NUM_THREADS];
         final AtomicInteger tripped = new AtomicInteger(0);
         final AtomicReference<Throwable> lastException = new AtomicReference<>(null);
@@ -167,7 +168,7 @@ public class MemoryCircuitBreakerTests extends ElasticsearchTestCase {
                 }
             }
         };
-        final BreakerSettings settings = new BreakerSettings(CircuitBreaker.Name.REQUEST, parentLimit + 1, 1.0);
+        final BreakerSettings settings = new BreakerSettings(CircuitBreaker.Name.REQUEST, childLimit, 1.0);
         final ChildMemoryCircuitBreaker breaker = new ChildMemoryCircuitBreaker(settings, logger,
                 (HierarchyCircuitBreakerService)service, CircuitBreaker.Name.REQUEST);
         breakerRef.set(breaker);
@@ -193,7 +194,7 @@ public class MemoryCircuitBreakerTests extends ElasticsearchTestCase {
         }
 
         logger.info("--> NUM_THREADS: [{}], BYTES_PER_THREAD: [{}], TOTAL_BYTES: [{}], PARENT_LIMIT: [{}], CHILD_LIMIT: [{}]",
-                NUM_THREADS, BYTES_PER_THREAD, (BYTES_PER_THREAD * NUM_THREADS), parentLimit, parentLimit + 1);
+                NUM_THREADS, BYTES_PER_THREAD, (BYTES_PER_THREAD * NUM_THREADS), parentLimit, childLimit);
 
         logger.info("--> starting threads...");
         for (Thread t : threads) {
@@ -211,6 +212,8 @@ public class MemoryCircuitBreakerTests extends ElasticsearchTestCase {
                 breaker.getUsed(), equalTo((long)parentLimit));
         assertThat("parent breaker was tripped exactly twice", parentTripped.get(), equalTo(2));
         assertThat("total breaker was tripped exactly twice", tripped.get(), equalTo(2));
+        assertThat("breaker total is expected value: " + parentLimit, breaker.getUsed(), equalTo((long)
+                parentLimit));
     }
 
     @Test