Replace assertions with more specific ones

This patch adds the already implemented assert(Not)Empty functions
instead of generic assert functions where assert(Not)Equal or
assertGreater(Equal) is used to check the state of a container.
This change makes the code and the error messages more readable,
therefore it improves maintainability a bit.

Change-Id: Ib7df910c7853141bcf2effd89f8fba864fc6bbb5
13 files changed