Fixed: move assignment not working for non-empty containers

[somaveszelovszki]

The move assignment was not working properly for some container types if the target is not empty. Affected container types: priority_queue, set, multiset. Solution: clear() called in these move assignment functions before adding the new elements to the target container. Unit tests modified for all containers types (even the ones that implemented the move assignment correctly) to prevent future regressions.

[semanticdiff-com[bot]]

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