Failing test to demonstrate need for extensionality rule

[tautschnig]

All existing tests rely on indexed access to arrays, which is covered by
the read-over-write axiom.

• 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

Merging #6824 (55f3861) into develop (bf66073) will increase coverage by 0.00%.
The diff coverage is 89.15%.

@@           Coverage Diff            @@
##           develop    #6824   +/-   ##
========================================
  Coverage    77.00%   77.01%           
========================================
  Files         1594     1594           
  Lines       184345   184417   +72     
========================================
+ Hits        141957   142021   +64     
- Misses       42388    42396    +8     

Impacted Files	Coverage Δ
...oto-instrument/contracts/instrument_spec_assigns.h	94.73% <ø> (ø)
src/goto-instrument/contracts/utils.h	83.01% <66.66%> (-16.99%)	⬇️
...o-instrument/contracts/instrument_spec_assigns.cpp	97.42% <100.00%> (+0.27%)	⬆️
src/ansi-c/anonymous_member.cpp	97.43% <0.00%> (+2.56%)	⬆️

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