Flaky test fix: allow ThreadPool to shutdown before asserting complet… #641

Job	Run time
Test isolated (ruby)	3m 41s
Test isolated (2.3)	3m 56s
Test without C extension	3m 17s
Tests: Ruby 2.6 - ubuntu	3m 36s
Tests: Ruby jruby - ubuntu	5m 51s
Tests: Ruby 2.5 - ubuntu	3m 34s
Tests: Ruby 3.1 - ubuntu	3m 21s
Tests: Ruby 2.4 - ubuntu	3m 30s
Tests: Ruby 3.2 - ubuntu	3m 18s
Tests: Ruby 3.0 - ubuntu	3m 20s
Tests: Ruby 2.3 - ubuntu	3m 34s
Tests: Ruby 3.4 - ubuntu	3m 21s
Tests: Ruby truffleruby - ubuntu	4m 17s
Tests: Ruby 2.7 - ubuntu	3m 30s
Tests: Ruby 3.3 - ubuntu	3m 21s
Tests: Ruby ruby - windows	4m 32s
Tests: Ruby 2.5 - ubuntu	3m 34s
Test isolated (2.3)	3m 56s
Tests: Ruby 2.6 - ubuntu	3m 36s
Tests: Ruby 3.2 - ubuntu	3m 18s
Tests: Ruby 2.3 - ubuntu	3m 34s
Tests: Ruby 3.1 - ubuntu	3m 21s
Tests: Ruby 3.0 - ubuntu	3m 20s
Tests: Ruby 2.4 - ubuntu	3m 30s
Tests: Ruby 2.7 - ubuntu	3m 30s
Test without C extension	3m 17s
Tests: Ruby jruby - ubuntu	5m 40s
Tests: Ruby 3.4 - ubuntu	3m 21s
Tests: Ruby 3.3 - ubuntu	3m 21s
Tests: Ruby ruby - windows	4m 32s
Tests: Ruby truffleruby - ubuntu	4m 17s
Test isolated (ruby)	3m 41s
	1h 59m 47s

Provide feedback