Limit string length to avoid spurious regression test failure

[tautschnig]

As seen in
https://github.com/diffblue/cbmc/actions/runs/4979949266/jobs/8933402303?pr=7413 we may end up with a counterexample too large to store on GitHub's runner. Limit the string length as the length is immaterial to the test.

• 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).
• n/a 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

Patch and project coverage have no change.

Comparison is base (0325f8e) 78.55% compared to head (367340b) 78.55%.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #7715   +/-   ##
========================================
  Coverage    78.55%   78.55%           
========================================
  Files         1686     1686           
  Lines       192930   192930           
========================================
  Hits        151562   151562           
  Misses       41368    41368           

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