Extend unit test of bits2expr/expr2bits with struct type

[tautschnig]

Structs with sub-byte bit fields demonstrate that big-endian ordering
also re-orders the bits within a byte. An executable variant of this
unit test was validated on mips (using qemu-mips), demonstrating the
same bit ordering.

No bug fixes/code changes required, this test just confirms that our
endianness interpretation matches actual hardware.

• 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 #6484 (7b2dcad) into develop (f5789d0) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop    #6484   +/-   ##
========================================
  Coverage    76.04%   76.04%           
========================================
  Files         1546     1546           
  Lines       165485   165500   +15     
========================================
+ Hits        125836   125851   +15     
  Misses       39649    39649           

Impacted Files	Coverage Δ
unit/util/simplify_expr.cpp	99.66% <100.00%> (+0.01%)	⬆️

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