interconnect: add DM test suite

interconnect: add DM test suite

Add a test suite exercising the whole lifetime and callbacks
of interconnect with a fake 5 providers with a split node graph.

The test suite checks the calculus are right and goes to the correct
nodes, and the lifetime of the node is correct.

Link: https://patch.msgid.link/20251120-topic-interconnect-next-v5-2-e8a82720da5d@linaro.org
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>