[QA] Tests for user endpoint in ocis multi instances

## Description
Add test to check user endpoint. It should return either the user is a member or a guest and the cross instance reference.

Set the envs:
```yaml
- OCIS_LDAP_CROSS_INSTANCE_REFERENCE_TEMPLATE: "{{.Username}}@{{.Instancename}}.owncloud.test"
- OCIS_LDAP_INSTANCE_URL_TEMPLATE: "https://{{.Instancename}}.owncloud.test"
```

Implemented in https://github.com/owncloud/ocis/pull/11925