feat: Implement all_ptaskgroups() and add more tests

[achimnol]

resolves #37

[codecov]

Codecov Report

Merging #38 (2ab9866) into main (446ffec) will decrease coverage by 0.14%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##             main      #38      +/-   ##
==========================================
- Coverage   65.87%   65.72%   -0.15%     
==========================================
  Files          16       16              
  Lines        1263     1275      +12     
==========================================
+ Hits          832      838       +6     
- Misses        431      437       +6     

Impacted Files	Coverage Δ
src/aiotools/taskgroup/persistent.py	0.00% <0.00%> (ø)
src/aiotools/taskgroup/persistent_compat.py	83.42% <100.00%> (+0.56%)	⬆️

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