Fix bug in SyntheticControl.get_plot_data

[drbenvincent]

• Closes Confusion over pred_hdi_upper_94 values #532
• Adds regression test
• Also changed the Makefile to get make test working robustly on my system

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📚 Documentation preview 📚: https://causalpy--540.org.readthedocs.build/en/540/

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.48%. Comparing base (a6ddda9) to head (fa5f5af).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #540      +/-   ##
==========================================
+ Coverage   95.43%   95.48%   +0.04%     
==========================================
  Files          28       29       +1     
  Lines        2586     2614      +28     
==========================================
+ Hits         2468     2496      +28     
  Misses        118      118              

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[juanitorduz]

Nice!