S3 fixture should report 404 on unknown bucket (#31782)

Today, `AmazonS3Fixture` returns 403 on attempts to access any inappropriate
bucket, whether known or otherwise. In fact, S3 reports 404 on nonexistent
buckets and 403 on inaccessible ones. This change enhances `AmazonS3Fixture` to
distinguish these cases.

plugins/repository-s3/src/test/java/org/elasticsearch/repositories/s3/AmazonS3Fixture.java

@@ -96,8 +96,13 @@ public class AmazonS3Fixture extends AbstractHttpFixture {
             if (handler != null) {
                 final String bucket = request.getParam("bucket");
                 if (bucket != null && permittedBucket.equals(bucket) == false) {
-                    // allow a null bucket to support bucket-free APIs
+                    // allow a null bucket to support the multi-object-delete API which
+                    // passes the bucket name in the host header instead of the URL.
+                    if (buckets.containsKey(bucket)) {
                         return newError(request.getId(), RestStatus.FORBIDDEN, "AccessDenied", "Bad bucket", "");
+                    } else {
+                        return newBucketNotFoundError(request.getId(), bucket);
+                    }
                 }
                 return handler.handle(request);
             } else {

plugins/repository-s3/src/test/resources/rest-api-spec/test/repository_s3/20_repository_permanent_credentials.yml

@@ -183,10 +183,6 @@ setup:
 ---
 "Register a repository with a non existing bucket":
 
-  - skip:
-      version: all
-      reason: to be fixed
-
   - do:
       catch: /repository_exception/
       snapshot.create_repository:

plugins/repository-s3/src/test/resources/rest-api-spec/test/repository_s3/30_repository_temporary_credentials.yml

@@ -183,10 +183,6 @@ setup:
 ---
 "Register a repository with a non existing bucket":
 
-  - skip:
-      version: all
-      reason: to be fixed
-
   - do:
       catch: /repository_exception/
       snapshot.create_repository: