LUCENE-8766: Further checks against race in test

lucene/monitor/src/test/org/apache/lucene/monitor/TestCachePurging.java

@@ -149,7 +149,9 @@ public class TestCachePurging extends MonitorTestBase {
       monitor.addQueryIndexUpdateListener(new MonitorUpdateListener() {
         @Override
         public void onPurge() {
-          latch.countDown();
+          // It can sometimes take a couple of purge runs to get everything in sync
+          if (monitor.getQueryCacheStats().cachedQueries == 99)
+            latch.countDown();
         }
       });