test(search): set Mtime on opensearch folder and root fixtures

The Mtime field is mapped as an OpenSearch `date`, which rejects an
empty value with `mapper_parsing_exception: cannot parse empty date`.
The folder and root fixtures had no Mtime, so serializing them to
`"Mtime": ""` made TestEngine_Purge/purge_resource_trees fail when the
document was indexed. Give both a valid RFC3339 Mtime, matching the
file fixture.

services/search/internal/opensearchtest/testdata/resource_folder.json

@@ -3,5 +3,6 @@
   "RootID" : "1$1!1",
   "ParentID" : "1$1!1",
   "Path" : "./parent d!r",
-  "Type" : 2
+  "Type" : 2,
+  "Mtime" : "2025-07-24T15:15:01.324093+02:00"
 }

services/search/internal/opensearchtest/testdata/resource_root.json

@@ -1,5 +1,6 @@
 {
   "ID" : "1$1!1",
   "RootID" : "1$1!1",
-  "Path" : "."
+  "Path" : ".",
+  "Mtime" : "2025-07-24T15:15:01.324093+02:00"
 }