Replace awaitBusy with assertBusy in atLeastDocsIndexed (elastic#38190)

Unlike assertBusy, awaitBusy does not retry if the code-block throws an
AssertionError. A refresh in atLeastDocsIndexed can fail because we call
this method while we are closing some node in FollowerFailOverIT.

x-pack/plugin/ccr/src/test/java/org/elasticsearch/xpack/CcrIntegTestCase.java

@@ -487,14 +487,15 @@ private Map<Integer, List<DocIdSeqNoAndTerm>> getDocIdAndSeqNos(InternalTestClus
         return docs;
     }
 
-    protected void atLeastDocsIndexed(Client client, String index, long numDocsReplicated) throws InterruptedException {
+    protected void atLeastDocsIndexed(Client client, String index, long numDocsReplicated) throws Exception {
         logger.info("waiting for at least [{}] documents to be indexed into index [{}]", numDocsReplicated, index);
-        awaitBusy(() -> {
+        assertBusy(() -> {
             refresh(client, index);
             SearchRequest request = new SearchRequest(index);
             request.source(new SearchSourceBuilder().size(0));
             SearchResponse response = client.search(request).actionGet();
-            return response.getHits().getTotalHits().value >= numDocsReplicated;
+            assertNotNull(response.getHits().getTotalHits());
+            assertThat(response.getHits().getTotalHits().value, greaterThanOrEqualTo(numDocsReplicated));
         }, 60, TimeUnit.SECONDS);
     }