Add pointer offset and object expression unit tests

[thomasspriggs]

This PR adds pointer offset and object expression unit tests.

• Each commit message has a non-empty body, explaining why the change was made.
• 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).
• My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[codecov]

Codecov Report

Merging #6803 (351cecb) into develop (69ac490) will increase coverage by 0.01%.
The diff coverage is 94.50%.

@@             Coverage Diff             @@
##           develop    #6803      +/-   ##
===========================================
+ Coverage    76.97%   76.99%   +0.01%     
===========================================
  Files         1593     1594       +1     
  Lines       184104   184263     +159     
===========================================
+ Hits        141718   141872     +154     
- Misses       42386    42391       +5     

Impacted Files	Coverage Δ
.../goto-instrument/accelerate/acceleration_utils.cpp	2.22% <0.00%> (ø)
src/goto-programs/interpreter_evaluate.cpp	33.88% <0.00%> (+0.11%)	⬆️
src/util/simplify_expr_class.h	90.47% <ø> (ø)
src/util/std_expr.h	93.18% <50.00%> (-0.48%)	⬇️
src/util/validate_expressions.cpp	93.75% <90.00%> (-1.71%)	⬇️
...o-instrument/contracts/instrument_spec_assigns.cpp	97.37% <100.00%> (ø)
src/goto-instrument/contracts/utils.cpp	89.36% <100.00%> (+0.11%)	⬆️
src/pointer-analysis/value_set.cpp	83.31% <100.00%> (ø)
src/solvers/flattening/bv_pointers.cpp	83.09% <100.00%> (+0.06%)	⬆️
src/solvers/smt2/smt2_conv.cpp	67.48% <100.00%> (ø)
... and 7 more

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