Check acknowledged responses in FsSearchableSnapshotsIT (elastic#59021)

Despite all my attempts I did not manage to reproduce issues like the ones 
described in elastic#58961. My guess is that the _mount request got retried at 
some point but I wasn't able to validate this assumption.

Still, the FsSearchableSnapshotsIT can be pretty disk heavy if a small 
random chunk size and a large number of documents is picked up in the 
tests. The parent class also does not verify the acknowledged status 
of some requests.

This commit lowers down the chunk size and number of docs in tests 
(this is extensively tests in unit tests) and also adds assertions on 
acknowledged responses.

Relates elastic#58961

x-pack/plugin/searchable-snapshots/qa/rest/src/test/java/org/elasticsearch/xpack/searchablesnapshots/rest/FsSearchableSnapshotsIT.java

@@ -6,7 +6,6 @@
 package org.elasticsearch.xpack.searchablesnapshots.rest;
 
 import org.elasticsearch.common.settings.Settings;
-import org.elasticsearch.common.unit.ByteSizeUnit;
 import org.elasticsearch.repositories.fs.FsRepository;
 import org.elasticsearch.xpack.searchablesnapshots.AbstractSearchableSnapshotsRestTestCase;
 
@@ -19,14 +18,6 @@ protected String repositoryType() {
 
     @Override
     protected Settings repositorySettings() {
-        final Settings.Builder settings = Settings.builder();
-        settings.put("location", System.getProperty("tests.path.repo"));
-        if (randomBoolean()) {
-            settings.put("compress", randomBoolean());
-        }
-        if (randomBoolean()) {
-            settings.put("chunk_size", randomIntBetween(100, 1000), ByteSizeUnit.BYTES);
-        }
-        return settings.build();
+        return Settings.builder().put("location", System.getProperty("tests.path.repo")).build();
     }
 }

x-pack/plugin/searchable-snapshots/src/test/java/org/elasticsearch/xpack/searchablesnapshots/AbstractSearchableSnapshotsRestTestCase.java

@@ -68,7 +68,7 @@ private void runSearchableSnapshotsTest(SearchableSnapshotsTestCaseBody testCase
         );
         ensureGreen(indexName);
 
-        final int numDocs = randomIntBetween(1, 10_000);
+        final int numDocs = randomIntBetween(1, 500);
         logger.info("indexing [{}] documents", numDocs);
 
         final StringBuilder bulkBody = new StringBuilder();
@@ -255,11 +255,7 @@ protected static void registerRepository(String repository, String type, boolean
         request.setJsonEntity(Strings.toString(new PutRepositoryRequest(repository).type(type).verify(verify).settings(settings)));
 
         final Response response = client().performRequest(request);
-        assertThat(
-            "Failed to create repository [" + repository + "] of type [" + type + "]: " + response,
-            response.getStatusLine().getStatusCode(),
-            equalTo(RestStatus.OK.getStatus())
-        );
+        assertAcked("Failed to create repository [" + repository + "] of type [" + type + "]: " + response, response);
     }
 
     protected static void createSnapshot(String repository, String snapshot, boolean waitForCompletion) throws IOException {
@@ -278,11 +274,7 @@ protected static void deleteSnapshot(String repository, String snapshot, boolean
         final Request request = new Request(HttpDelete.METHOD_NAME, "_snapshot/" + repository + '/' + snapshot);
         try {
             final Response response = client().performRequest(request);
-            assertThat(
-                "Failed to delete snapshot [" + snapshot + "] in repository [" + repository + "]: " + response,
-                response.getStatusLine().getStatusCode(),
-                equalTo(RestStatus.OK.getStatus())
-            );
+            assertAcked("Failed to delete snapshot [" + snapshot + "] in repository [" + repository + "]: " + response, response);
         } catch (IOException e) {
             if (ignoreMissing && e instanceof ResponseException) {
                 Response response = ((ResponseException) e).getResponse();
@@ -324,6 +316,22 @@ protected static void mountSnapshot(
         );
     }
 
+    protected static void deleteIndex(String index) throws IOException {
+        final Response response = client().performRequest(new Request("DELETE", "/" + index));
+        assertAcked("Fail to delete index [" + index + ']', response);
+    }
+
+    private static void assertAcked(String message, Response response) throws IOException {
+        final int responseStatusCode = response.getStatusLine().getStatusCode();
+        assertThat(
+            message + ": expecting response code [200] but got [" + responseStatusCode + ']',
+            responseStatusCode,
+            equalTo(RestStatus.OK.getStatus())
+        );
+        final Map<String, Object> responseAsMap = responseAsMap(response);
+        assertThat(message + ": response is not acknowledged", extractValue(responseAsMap, "acknowledged"), equalTo(Boolean.TRUE));
+    }
+
     protected static void forceMerge(String index, boolean onlyExpungeDeletes, boolean flush) throws IOException {
         final Request request = new Request(HttpPost.METHOD_NAME, '/' + index + "/_forcemerge");
         request.addParameter("only_expunge_deletes", Boolean.toString(onlyExpungeDeletes));