Add the smt-backend tags to tests that use --z3

[martin-cs]

This means that the core tests will run cleanly without z3 on the path.

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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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[codecov]

Codecov Report

Merging #6181 (7486222) into develop (0002950) will increase coverage by 7.84%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop    #6181      +/-   ##
===========================================
+ Coverage    67.40%   75.25%   +7.84%     
===========================================
  Files         1157     1454     +297     
  Lines        95236   160796   +65560     
===========================================
+ Hits         64197   121006   +56809     
- Misses       31039    39790    +8751     

Flag	Coverage Δ
cproversmt2	?
regression	?
unit	?

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/util/string_container.cpp	52.94% <0.00%> (-47.06%)	⬇️
src/ansi-c/gcc_types.cpp	41.86% <0.00%> (-46.38%)	⬇️
src/solvers/prop/prop.cpp	42.85% <0.00%> (-41.76%)	⬇️
src/solvers/flattening/boolbv_member.cpp	53.65% <0.00%> (-38.65%)	⬇️
src/cpp/cpp_storage_spec.cpp	65.00% <0.00%> (-35.00%)	⬇️
src/util/cmdline.h	66.66% <0.00%> (-33.34%)	⬇️
src/solvers/strings/array_pool.h	66.66% <0.00%> (-33.34%)	⬇️
src/solvers/strings/string_refinement.h	66.66% <0.00%> (-33.34%)	⬇️
...rs/strings/string_concatenation_builtin_function.h	0.00% <0.00%> (-33.34%)	⬇️
src/ansi-c/c_typecheck_base.cpp	50.54% <0.00%> (-30.35%)	⬇️
... and 1433 more

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[martin-cs]

The code coverage results appear to be way off.

[SaswatPadhi]

Good catch!

Thanks for fixing this.