Allow CapacityLimiter to have zero total_tokens

[AbduazizZiyodov]

Closes #3243

Hi, I've only changed some existing tests & added one assert statement which checks total_tokens in case of zero. What kind of other tests should be included, if any needed ?

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00000%. Comparing base (77dd921) to head (2f4c529).

Additional details and impacted files

@@               Coverage Diff               @@
##                 main        #3321   +/-   ##
===============================================
  Coverage   100.00000%   100.00000%           
===============================================
  Files             125          125           
  Lines           19251        19252    +1     
  Branches         1304         1304           
===============================================
+ Hits            19251        19252    +1     

Files with missing lines	Coverage Δ
src/trio/_sync.py	100.00000% <100.00000%> (ø)
src/trio/_tests/test_sync.py	100.00000% <100.00000%> (ø)

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[A5rocks]

Thanks for the PR! Could you:

• add a newsfragment
• add a test for something like:
  • create and use a capacity limiter with a limit of, say, 5
  • set the limit to 0
  • make sure it stops letting anything through
  • set the limit up to, say, 5
  • make sure it let everything through
• maybe also a test that a 0 limit capacity limiter doesn't let any tasks through? This is covered by the above but it's better to be extra sure.

If you can think of any case you're unsure works, that's what you would make a test case for!

[AbduazizZiyodov]

@A5rocks of course, I'll let you know then 🤝