Add unit test of object size tracking for new SMT backend.

[thomasspriggs]

This PR adds unit tests of object size tracking for new SMT backend. We were previously missing unit tests for the tracking of object sizes. Note that this test includes tracking for when object sizes are based on symbols rather than statically defined. This is used for dynamically sized objects such as those pointed to by the return value of malloc.

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• Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
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[NlightNFotis]

Don't worry about the failing CI, it's a network issue on codecov's end.