let unit tests run faster

[HerbertKoelman]

more here

[codecov]

Codecov Report

Merging #104 into develop will increase coverage by 0.57%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##           develop     #104      +/-   ##
===========================================
+ Coverage    88.15%   88.72%   +0.57%     
===========================================
  Files           16       16              
  Lines          692      692              
===========================================
+ Hits           610      614       +4     
+ Misses          82       78       -4

Impacted Files	Coverage Δ
tests/thread_tests.cpp	85.29% <100%> (ø)	⬆️
tests/synchronized_queue_tests.cpp	97.53% <100%> (+2.46%)	⬆️
tests/abstract_thread_tests.cpp	95.74% <100%> (ø)	⬆️
include/pthread/sync_queue.hpp	95% <0%> (+10%)	⬆️

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