Fix printing of SMT data structures in failed unit tests

[thomasspriggs]

The operator needed for the printing is defined in smt_to_smt2_string.cpp, but it needs to be forward declared for the catch framework to find and use it, instead of printing SMT data structures as {?}.

A regression test of a failing unit test is included in this PR to ensure that this functionality for fault finding of failing unit tests works as intended.

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[codecov]

Codecov Report

Base: 78.03% // Head: 78.04% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (c110dcf) compared to base (21e3260).
Patch coverage: 94.73% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #7287   +/-   ##
========================================
  Coverage    78.03%   78.04%           
========================================
  Files         1624     1624           
  Lines       187405   187414    +9     
========================================
+ Hits        146249   146262   +13     
+ Misses       41156    41152    -4     

Impacted Files	Coverage Δ
src/solvers/flattening/boolbv_width.cpp	73.80% <0.00%> (ø)
src/ansi-c/expr2c.cpp	67.35% <100.00%> (+0.03%)	⬆️
src/goto-programs/goto_clean_expr.cpp	99.02% <100.00%> (ø)
src/goto-programs/goto_convert_side_effect.cpp	94.50% <100.00%> (+0.02%)	⬆️
src/goto-symex/goto_symex.cpp	98.64% <100.00%> (ø)
...it/solvers/smt2_incremental/smt_to_smt2_string.cpp	100.00% <100.00%> (ø)
unit/catch/catch.hpp	42.91% <0.00%> (+0.04%)	⬆️
...rc/solvers/smt2_incremental/smt_to_smt2_string.cpp	96.73% <0.00%> (+1.08%)	⬆️

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