tests: reorganize infopipe tests by interface

Group the infopipe tests into logical sections based on the D-Bus
interface they exercise:
- Infopipe (root object)
- Domains
- Users
- Groups
- Mixed/Combined interfaces

This is a pure reorganization with no logic changes to improve test
maintainability and readability.

Reviewed-by: Jakub Vávra <jvavra@redhat.com>
Reviewed-by: Shridhar Gadekar <sgadekar@redhat.com>

Author: aborah-sudo