Keep unary in individuals test

[hyanwong]

Fixes #1154

This is rebased off #1186 and includes (cherry-picked) #1190, so I can wait until those are sorted before pushing this one.

PR Checklist:

• Tests that fully cover new/changed functionality.
• Documentation including tutorial content if appropriate.
• Changelogs, if there are API changes.

[codecov]

Codecov Report

Merging #1191 (f6fcd87) into main (028997b) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #1191   +/-   ##
=======================================
  Coverage   93.72%   93.72%           
=======================================
  Files          26       26           
  Lines       21511    21514    +3     
  Branches      904      904           
=======================================
+ Hits        20161    20164    +3     
  Misses       1312     1312           
  Partials       38       38           

Flag	Coverage Δ
c-tests	92.50% <100.00%> (ø)
lwt-tests	92.97% <ø> (ø)
python-c-tests	94.91% <100.00%> (+<0.01%)	⬆️
python-tests	98.63% <ø> (ø)

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Impacted Files	Coverage Δ
python/tskit/tables.py	99.60% <ø> (ø)
python/tskit/trees.py	97.42% <ø> (ø)
c/tskit/tables.c	90.83% <100.00%> (ø)
python/_tskitmodule.c	91.43% <100.00%> (+<0.01%)	⬆️

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[hyanwong]

Moved to #1190