Unit tests: verify complementary pair detection in AND/OR

[tautschnig]

Add unit tests confirming that the simplifier detects complementary pairs in boolean AND/OR expressions:
A || !A -> true
A && !A -> false
A || (B || !A) -> true (after flattening by join_operands)

This is already handled by the existing 'search for a and !a' loop in simplify_boolean(). The tests document and protect this behavior.

• 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.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.50%. Comparing base (a6531d4) to head (a5834d1).
⚠️ Report is 2 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #8982   +/-   ##
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  Coverage    80.50%   80.50%           
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  Files         1704     1704           
  Lines       188796   188816   +20     
  Branches        73       73           
========================================
+ Hits        151993   152013   +20     
  Misses       36803    36803           

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[kroening]

We'll want a variant of namespacet that's simply empty.