Exercise multi-part uploads in S3 repo analysis tests (#125669)

Extends the max blob size to 10MiB, and sets the buffer size to 5MiB, to
ensure that sometimes the S3 repo analysis tests will create blobs using
the multipart upload flow.

x-pack/plugin/snapshot-repo-test-kit/qa/s3/src/javaRestTest/java/org/elasticsearch/repositories/blobstore/testkit/analyze/S3RepositoryAnalysisRestIT.java

@@ -9,6 +9,7 @@ package org.elasticsearch.repositories.blobstore.testkit.analyze;
 import fixture.s3.S3HttpFixture;
 
 import org.elasticsearch.common.settings.Settings;
+import org.elasticsearch.common.unit.ByteSizeValue;
 import org.elasticsearch.test.cluster.ElasticsearchCluster;
 import org.elasticsearch.test.cluster.local.distribution.DistributionType;
 import org.junit.ClassRule;
@@ -59,6 +60,7 @@ public class S3RepositoryAnalysisRestIT extends AbstractRepositoryAnalysisRestTe
             .put("bucket", bucket)
             .put("base_path", basePath)
             .put("delete_objects_max_size", between(1, 1000))
+            .put("buffer_size", ByteSizeValue.ofMb(5)) // so some uploads are multipart ones
             .build();
     }
 }

x-pack/plugin/snapshot-repo-test-kit/src/test/java/org/elasticsearch/repositories/blobstore/testkit/analyze/AbstractRepositoryAnalysisRestTestCase.java

@@ -29,7 +29,7 @@ public abstract class AbstractRepositoryAnalysisRestTestCase extends ESRestTestC
         final Request request = new Request(HttpPost.METHOD_NAME, "/_snapshot/" + repository + "/_analyze");
         request.addParameter("blob_count", "10");
         request.addParameter("concurrency", "4");
-        request.addParameter("max_blob_size", "1mb");
+        request.addParameter("max_blob_size", randomFrom("1mb", "10mb"));
         request.addParameter("timeout", "120s");
         request.addParameter("seed", Long.toString(randomLong()));
         assertOK(client().performRequest(request));