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/posix/posix.go

@@ -3472,8 +3472,9 @@ func (p *Posix) ListMultipartUploads(ctx context.Context, mpu *s3.ListMultipartU
 		if prefix != "" && !strings.HasPrefix(objectName, prefix) {
 			continue
 		}
-		// filter by keyMarker
-		if keyMarker != "" && objectName <= keyMarker {
+		// filter by keyMarker, keeping the uploads of the key-marker
+		// itself when an upload-id-marker is set
+		if keyMarker != "" && (objectName < keyMarker || (objectName == keyMarker && uploadIDMarker == "")) {
 			continue
 		}