The major point of NMDedupMultiIndex is that it can de-duplicate
the objects. It thus makes sense the everybody is using the same
instance. Make the multi-idx instance of NMPlatform configurable.

This is not used outside of unit tests, because the daemon currently
always creates one platform instance and everybody then re-uses the
instance of the platform.

While this is (currently) only used by tests, and that the performance
optimization of de-duplicating is irrelevant for tests, this is still
useful. The test can then check whether two separate NMPlatform objects
shared the same instance and whether it was de-duplicated.