Add tests for ClosureDiagram when the fixed point subalgebra is a torus

[fingolfin]

ClosureDiagram ran into a "no method found" error when the fixed point subalgebra of the automorphism is a torus (#8). Commit a1322ca fixed that, but there is no test for it: the two existing ClosureDiagram examples in the manual do not cover this case.

This adds tst/ClosureDiagram.tst with the two examples from #8.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.29%. Comparing base (d4a7e49) to head (c2d1b7a).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #11      +/-   ##
==========================================
+ Coverage   97.15%   97.29%   +0.14%     
==========================================
  Files          16       16              
  Lines       92584    92584              
==========================================
+ Hits        89950    90082     +132     
+ Misses       2634     2502     -132     

see 3 files with indirect coverage changes

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