[TEST] Synchronize searcher list in IndexShardTests

It's possible to check the list size, then attempt to remove a searcher and
throw an IndexOutOfBoundsException due to multiple threads.

Resolves elastic#27651

server/src/test/java/org/elasticsearch/index/shard/IndexShardTests.java

@@ -2781,9 +2781,14 @@ public void testSegmentMemoryTrackedWithRandomSearchers() throws Exception {
 
                         if (randomBoolean() && searchers.size() > 1) {
                             // Close one of the searchers at random
-                            Engine.Searcher searcher = searchers.remove(0);
-                            logger.debug("--> {} closing searcher {}", threadName, searcher.source());
-                            IOUtils.close(searcher);
+                            synchronized (searchers) {
+                                // re-check because it could have decremented after the check
+                                if (searchers.size() > 1) {
+                                    Engine.Searcher searcher = searchers.remove(0);
+                                    logger.debug("--> {} closing searcher {}", threadName, searcher.source());
+                                    IOUtils.close(searcher);
+                                }
+                            }
                         }
                     } catch (Exception e) {
                         logger.warn("--> got exception: ", e);