[Tests] Ignore indices when testing global state for snapshot/restore (#71324)

The failed test is to ensure opreator-only settings can be snapshot and
restored by regular users when the operator privileges feature is disabled. It
only cares about cluster settings, not any indices. But it currently includes
every index into its snapshot. This creates a problem because the indices
cannot be restored unless the original ones are deleted or closed and this
results in a 500 error.

This PR fixes the issue by excluding all indices from the snapshot/restore
process. It is the simplest viable fix since the test only cares about cluster
settings.

x-pack/plugin/src/yamlRestTest/resources/rest-api-spec/test/snapshot/20_operator_privileges_disabled.yml

@@ -42,13 +42,14 @@ teardown:
         snapshot: test_snapshot_2
         wait_for_completion: true
         body: |
-          { "include_global_state": true }
+          { "include_global_state": true, "indices": "-*" }
 
   - match: { snapshot.snapshot: test_snapshot_2 }
   - match: { snapshot.state : SUCCESS }
   - is_true: snapshot.include_global_state
   - is_true: snapshot.version
   - gt: { snapshot.version_id: 0}
+  - length: { snapshot.indices: 0 }
 
   - do:
       cluster.put_settings: