Load so files from a given directory for tests

[eaydingol]

DESCRIPTION: PR description that will go into the change log, up to 78 characters

Adds support for loading citus and columnar so files from a different directory. Builds on open https://github.com/citusdata/citus/pull/8361

usage: CITUSLIBDIR=/tmp/opt/citus_versions/v13.2.1 make check-minimal CITUSLIBDIR=/tmp/opt/citus_versions/v13.2.1 citus_tests/run_test.py columnar_create

[codecov[bot]]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 88.95%. Comparing base (662b724) to head (2f349ce). :warning: Report is 8 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8369      +/-   ##
==========================================
- Coverage   88.95%   88.95%   -0.01%     
==========================================
  Files         287      287              
  Lines       63151    63145       -6     
  Branches     7942     7934       -8     
==========================================
- Hits        56176    56170       -6     
+ Misses       4667     4665       -2     
- Partials     2308     2310       +2     

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