Fix cancellation in fs2 3

[RafalSumislawski]

This fixes #409 for fs2-3. The fix is trivial in this case because CE3 provides interruptibleMany. At the moment I don't know how to fix it for other variants of the library.

I also reproduced the issue using an existing test (it used to pass because cancel was called fast, before the uninterruptible effect started executing) and added a second similar test that checks specifically terminating the sub-process by releasing the Resource.

[vigoo]

Thank you! I will check the other variants.

[codecov]

Codecov Report

Base: 84% // Head: 84% // Decreases project coverage by -0% ⚠️

Coverage data is based on head (e60d340) compared to base (3d803d8).
Patch coverage: 100% of modified lines in pull request are covered.

Additional details and impacted files

@@          Coverage Diff          @@
##           master   #410   +/-   ##
=====================================
- Coverage      84%    84%   -1%     
=====================================
  Files          14     14           
  Lines         652    652           
  Branches        6      5    -1     
=====================================
- Hits          550    549    -1     
- Misses        102    103    +1     

Impacted Files	Coverage Δ
.../src/main/scala/io/github/vigoo/prox/ProxFS2.scala	90% <100%> (ø)
.../src/main/scala/io/github/vigoo/prox/ProxFS2.scala	88% <0%> (-3%)	⬇️

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