fix: accept upload-id-marker together with key-marker in ListMultipartUploads

The posix and azure backends dropped every upload with a key equal to
the key-marker before handing the list to the lister, which then looked
for the upload-id-marker under the next key. An upload-id-marker that
belongs to the key-marker could never be found, so the request failed
with "Invalid uploadId marker", including when a client paged with the
NextKeyMarker and NextUploadIdMarker the gateway returned.

Keep the uploads of the key-marker itself when an upload-id-marker is
set, so the listing continues right after the given upload, as S3 does.
Enable the REST test that covers it.

Fixes #2149

backend/azure/azure.go

@@ -1760,7 +1760,7 @@ func (az *Azure) ListMultipartUploads(ctx context.Context, input *s3.ListMultipa
 			if !ok {
 				continue
 			}
-			if keyMarker != "" && *key <= keyMarker {
+			if keyMarker != "" && (*key < keyMarker || (*key == keyMarker && uploadIDMarker == "")) {
 				continue
 			}
 			if prefix != "" && !strings.HasPrefix(*key, prefix) {