Adjust for genno 1.16

[khaeru]

genno 1.16 slightly changed the messages on unparseable pint units, to be more informative. This PR adjusts one test to check for the newer message.

How to review

Read the diff and note that the CI checks all pass.

PR checklist

• [x] Continuous integration checks all ✅
• [x] Add or expand tests; coverage checks both ✅
• ~Add, expand, or update documentation.~ N/A
• ~Update release notes.~

[codecov[bot]]

Codecov Report

Merging #480 (d2771c8) into main (41b6290) will not change coverage. The diff coverage is 100.0%.

@@          Coverage Diff          @@
##            main    #480   +/-   ##
=====================================
  Coverage   98.5%   98.5%           
=====================================
  Files         42      42           
  Lines       4489    4489           
=====================================
  Hits        4425    4425           
  Misses        64      64           

Impacted Files	Coverage Δ
ixmp/tests/reporting/test_reporter.py	100.0% <100.0%> (ø)