Tests for indexing meshes

[pt331]

Description

Integration tests for #7152

[codecov]

Codecov Report

❌ Patch coverage is 62.96296% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.86%. Comparing base (a77af27) to head (a2dc89a).

Files with missing lines	Patch %	Lines
lib/iris/mesh/components.py	62.96%	6 Missing and 4 partials ⚠️

Additional details and impacted files

@@                  Coverage Diff                  @@
##           FEATURE_index_set    #7218      +/-   ##
=====================================================
+ Coverage              89.27%   89.86%   +0.59%     
=====================================================
  Files                     92       92              
  Lines                  25555    25573      +18     
  Branches                4762     4768       +6     
=====================================================
+ Hits                   22813    22980     +167     
+ Misses                  1945     1789     -156     
- Partials                 797      804       +7     

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[trexfeathers]

Thanks @pt331 !