Test and improve .report

[khaeru]

Will close #118.

How to review

TBA

PR checklist

• Continuous integration checks all ✅
• Add or expand tests; coverage checks both ✅
• Add, expand, or update documentation.
• Update doc/whatsnew.

[codecov]

Codecov Report

Merging #119 (f65d250) into main (2f62b67) will increase coverage by 1.5%.
The diff coverage is 97.4%.

@@           Coverage Diff           @@
##            main    #119     +/-   ##
=======================================
+ Coverage   66.9%   68.5%   +1.5%     
=======================================
  Files         70      71      +1     
  Lines       4635    4669     +34     
=======================================
+ Hits        3104    3200     +96     
+ Misses      1531    1469     -62     

Files Changed	Coverage Δ
message_ix_models/tests/test_cli.py	100.0% <ø> (ø)
message_ix_models/report/computations.py	57.3% <80.0%> (+18.3%)	⬆️
...essage_ix_models/tests/report/test_computations.py	100.0% <100.0%> (ø)
message_ix_models/tests/report/test_sim.py	100.0% <100.0%> (ø)

... and 2 files with indirect coverage changes

[khaeru]

Changes picked up in #134 and others.