Fix up tests of pyam compatibility

[znicholls]

Pull request

Please confirm that this pull request has done the following:

• To xarray #151
• Tests added
• Documentation added (where applicable)
• Example added (either to an existing notebook or as a new notebook, where applicable)
• Description in CHANGELOG.rst added

Adding to CHANGELOG.rst

Please add a single line in the changelog notes similar to one of the following:

- (`#XX <https://github.com/openscm/scmdata/pull/XX>`_) Added feature which does something
- (`#XX <https://github.com/openscm/scmdata/pull/XX>`_) Fixed bug identified in (`#YY <https://github.com/openscm/scmdata/issues/YY>`_)

[codecov]

Codecov Report

Merging #152 (6524217) into master (33036a2) will increase coverage by 0.14%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #152      +/-   ##
==========================================
+ Coverage   94.29%   94.44%   +0.14%     
==========================================
  Files          15       16       +1     
  Lines        1614     1656      +42     
  Branches      312      319       +7     
==========================================
+ Hits         1522     1564      +42     
  Misses         75       75              
  Partials       17       17              

Impacted Files	Coverage Δ
src/scmdata/_xarray.py	100.00% <100.00%> (ø)
src/scmdata/netcdf.py	100.00% <100.00%> (ø)
src/scmdata/run.py	95.16% <100.00%> (+0.01%)	⬆️

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