Add regression test to ascertain struct hack support

[tautschnig]

When a struct with a C90-style flexible array member (e.g., char data[1], aka "struct hack") was allocated with extra space using malloc(sizeof(struct Foo) + sizeof(char)*2), CBMC before 78839a9 incorrectly failed assertions when accessing elements beyond the declared array size.

Co-authored-by: Kiro autonomous agent

Fixes: #5213

• 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.
✅ Project coverage is 80.01%. Comparing base (76dd20f) to head (109e145).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #8801   +/-   ##
========================================
  Coverage    80.00%   80.01%           
========================================
  Files         1700     1700           
  Lines       188249   188249           
  Branches        73       73           
========================================
+ Hits        150615   150632   +17     
+ Misses       37634    37617   -17     

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