Limit counterexample size to speed up regression tests

[tautschnig]

These regression tests only require 1-2 seconds of solving time, but may produce counterexample traces that are greater than 8 GB on disk. Limit the maximum string length (in none of the tests does the length even matter) to avoid such artificial blow-up. Also, these tests were possibly a lot slower with other solvers, simply because these would produce different models. Before this PR, we had:

• assume-inputs-non-null/string_assume.desc:
  • cadical: 165 seconds
  • minisat: 0 seconds
• context-include-exclude/test_exclude_from_include.desc:
  • cadical: 796 seconds
  • minisat: 1 seconds
• context-include-exclude/test_include.desc:
  • cadical: 802 seconds
  • minisat: 1 seconds
• exceptions29/test.desc:
  • cadical: 168 seconds
  • minisat: 0 seconds
    when the actual solver time was less than 2 seconds.

• Each commit message has a non-empty body, explaining why the change was made.
• n/a Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• n/a 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.
• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[codecov]

Codecov Report

Base: 78.49% // Head: 78.46% // Decreases project coverage by -0.03% ⚠️

Coverage data is based on head (d9b0744) compared to base (1396782).
Patch has no changes to coverable lines.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #7539      +/-   ##
===========================================
- Coverage    78.49%   78.46%   -0.03%     
===========================================
  Files         1667     1667              
  Lines       191431   191431              
===========================================
- Hits        150255   150214      -41     
- Misses       41176    41217      +41     

Impacted Files	Coverage Δ
src/big-int/bigint.hh	89.47% <0.00%> (-7.90%)	⬇️
src/big-int/bigint.cc	83.35% <0.00%> (-5.65%)	⬇️

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