miniBDD test: support lselect

[tautschnig]

We can support the select operation, it just wasn't implemented.

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• 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).
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[codecov]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 79.67%. Comparing base (a8861ed) to head (d71cc12).

Files	Patch %	Lines
unit/solvers/bdd/miniBDD/miniBDD.cpp	0.00%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8240      +/-   ##
===========================================
- Coverage    79.67%   79.67%   -0.01%     
===========================================
  Files         1682     1682              
  Lines       195490   195491       +1     
===========================================
- Hits        155765   155749      -16     
- Misses       39725    39742      +17     

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