Disable intermittently-failing different_hierarchies test

[greenc-FNAL]

See #359

[Copilot]

Pull request overview

Disables the intermittently failing different_hierarchies unit test in the CMake-based test registration, pending the fix tracked in issue #359 (“Change fold-result caching to use multilayer_join_node”).

Changes:

• Removes different_hierarchies from the active cet_test(...) list by wrapping its registration in an always-false CMake conditional.
• Adds an inline comment referencing the tracking issue for re-enabling the test once resolved.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

@@            Coverage Diff             @@
##             main     #488      +/-   ##
==========================================
- Coverage   85.52%   85.44%   -0.09%     
==========================================
  Files         142      142              
  Lines        3586     3586              
  Branches      615      615              
==========================================
- Hits         3067     3064       -3     
- Misses        310      311       +1     
- Partials      209      211       +2     

Flag	Coverage Δ
unittests	85.44% <ø> (-0.09%)	⬇️

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