interconnect: Add kunit tests for core functionality

interconnect: Add kunit tests for core functionality

The interconnect framework currently lacks in-tree unit tests to verify
the core logic in isolation. This makes it difficult to validate
regression stability when modifying the provider/consumer APIs or
aggregation logic.

Introduce a kunit test suite that verifies the fundamental behavior of
the subsystem. The tests cover:
- Provider API (node creation, linking, topology construction).
- Consumer API (path enabling/disabling, bandwidth requests).
- Standard aggregation logic (accumulating bandwidth across links).
- Bulk operations for setting bandwidth on multiple paths.

The suite simulates a simple SoC topology with multiple masters and a
shared bus to validate traffic aggregation behavior in a controlled
software environment, without requiring specific hardware or Device
Tree support.

Signed-off-by: Kuan-Wei Chiu <visitorckw@gmail.com>
Link: https://lore.kernel.org/r/20260110184309.906735-1-visitorckw@gmail.com
Signed-off-by: Georgi Djakov <djakov@kernel.org>