TST: correct skip_plot marker

[rcomer]

Description

Pytest just fixed a bug whereby the --strict-markers option was being ignored pytest-dev/pytest#14443

#7162 is therefore failing because skipIf is an unknown marker. I assume it should be skipif. I attempted to make a Matplotlib-less dev environment to check that this now works, but found that you can't even import iris.cube without Cartopy being installed, and Matplotlib has been a required dependency of Cartopy for a while now.

    import iris.cube
lib/iris/cube.py:42: in <module>
    import iris._merge
lib/iris/_merge.py:28: in <module>
    import iris.coords
lib/iris/coords.py:40: in <module>
    import iris.util
lib/iris/util.py:40: in <module>
    from iris.coord_systems import GeogCS
lib/iris/coord_systems.py:19: in <module>
    import cartopy.crs as ccrs
E   ModuleNotFoundError: No module named 'cartopy'

Checklist

Important

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• Included a What's New entry
• Incorporated type hints in any changed code
• Checked if tests need updating
• Checked if benchmarks need updating
• Checked if documentation needs updating
  • Docstrings (we love code examples!)
  • User Manual pages
• Checked if dependencies need updating
• Confirmed that the GitHub 'checks' on this PR are passing ✅
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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.15%. Comparing base (3f0dd8d) to head (335eff8).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7163   +/-   ##
=======================================
  Coverage   90.15%   90.15%           
=======================================
  Files          91       91           
  Lines       24983    24983           
  Branches     4685     4685           
=======================================
  Hits        22524    22524           
  Misses       1682     1682           
  Partials      777      777           

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[trexfeathers]

Thanks @rcomer!