Fix testPendingTasks (#42922)

Fixes a race in the test which can be reliably reproduced by adding Thread.sleep(100) to the end of
IndicesService.processPendingDeletes

Closes #18747

server/src/test/java/org/elasticsearch/indices/IndicesServiceTests.java

@@ -330,8 +330,10 @@ public class IndicesServiceTests extends ESSingleNodeTestCase {
                 fail(e.getMessage());
             }
         });
+        assertBusy(() -> {
             assertThat(indicesService.hasUncompletedPendingDeletes(), equalTo(hasBogus)); // "bogus" index has not been removed
             assertFalse(shardPath.exists());
+        });
     }
 
     public void testVerifyIfIndexContentDeleted() throws Exception {