Add tests of existing smt2irep functionality

[thomasspriggs]

To ensure it is working as expected when reused/built on top of.

• 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 #6383 (c2c6c96) into develop (3c9339a) will increase coverage by 0.01%.
The diff coverage is 83.09%.

@@             Coverage Diff             @@
##           develop    #6383      +/-   ##
===========================================
+ Coverage    75.91%   75.92%   +0.01%     
===========================================
  Files         1517     1518       +1     
  Lines       163936   164007      +71     
===========================================
+ Hits        124451   124526      +75     
+ Misses       39485    39481       -4     

Impacted Files	Coverage Δ
unit/solvers/smt2/smt2irep.cpp	83.09% <83.09%> (ø)
src/solvers/smt2/smt2_tokenizer.cpp	97.70% <0.00%> (+9.16%)	⬆️
src/solvers/smt2/smt2irep.cpp	100.00% <0.00%> (+10.25%)	⬆️

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[TGWDB]

Looks fine on the (discussed) understanding that this is testing a subset of the functionality and further tests for smt2irep will be added as more parsing is required for the new SMT2 backend.