fix nondeterministic hex_trace test

[kroening]

The cbmc/hex_trace test pattern relies on a particular value (0) for an uninitialized local variable. This removes the pattern.

See
https://github.com/diffblue/cbmc/actions/runs/6672135313/job/18269948126?pr=7979 as an exemplar where this was triggered.

• 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

All modified and coverable lines are covered by tests ✅

Comparison is base (5341b44) 79.08% compared to head (39513a3) 79.08%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8121      +/-   ##
===========================================
- Coverage    79.08%   79.08%   -0.01%     
===========================================
  Files         1696     1696              
  Lines       196429   196429              
===========================================
- Hits        155346   155343       -3     
- Misses       41083    41086       +3     

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