Remove test numbering, as an experiment.

[pp-mo]

WIP

This is just to check whether the problems discussed at #5037 are still a reality -- i.e. whether the "test numbering" strategy is still needed.
Even if it is no longer required, I don't actually propose that we should remove the test numbers,
but at least in that case we could happily close #5307 !

[codecov]

Codecov Report

Attention: 37 lines in your changes are missing coverage. Please review.

Comparison is base (0b54d58) 89.25% compared to head (6afda2d) 89.45%.
Report is 142 commits behind head on FEATURE_split_attrs.

Files	Patch %	Lines
lib/iris/fileformats/netcdf/saver.py	91.06%	12 Missing and 4 partials ⚠️
lib/iris/fileformats/netcdf/_thread_safe_nc.py	79.48%	7 Missing and 1 partial ⚠️
lib/iris/__init__.py	70.00%	2 Missing and 1 partial ⚠️
lib/iris/analysis/__init__.py	96.80%	1 Missing and 2 partials ⚠️
lib/iris/fileformats/netcdf/_dask_locks.py	92.68%	2 Missing and 1 partial ⚠️
lib/iris/_concatenate.py	98.57%	0 Missing and 1 partial ⚠️
lib/iris/cube.py	99.10%	0 Missing and 1 partial ⚠️
lib/iris/fileformats/name_loaders.py	0.00%	1 Missing ⚠️
lib/iris/quickplot.py	92.30%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@                   Coverage Diff                   @@
##           FEATURE_split_attrs    #5584      +/-   ##
=======================================================
+ Coverage                89.25%   89.45%   +0.20%     
=======================================================
  Files                       88       90       +2     
  Lines                    22197    22603     +406     
  Branches                  4858     5437     +579     
=======================================================
+ Hits                     19811    20220     +409     
+ Misses                    1641     1638       -3     
  Partials                   745      745              

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