Add goroutine leak detection to unit test

[gammazero]

Check that shutdown works correctly by checking for any goroutines that are still running.

[codecov-commenter]

Codecov Report

Merging #152 (0966ab8) into main (c38b9f6) will decrease coverage by 0.81%.
Report is 1 commits behind head on main.
The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #152      +/-   ##
==========================================
- Coverage   31.83%   31.02%   -0.81%     
==========================================
  Files           5        5              
  Lines         735      738       +3     
==========================================
- Hits          234      229       -5     
- Misses        466      474       +8     
  Partials       35       35              

see 1 file with indirect coverage changes