Flaky test fix: allow ThreadPool to shutdown before asserting completed_task_count #639

Job	Run time
Test isolated (ruby)	3m 39s
Test without C extension	3m 21s
Test isolated (2.3)	3m 56s
Tests: Ruby 2.7 - ubuntu	3m 30s
Tests: Ruby 2.5 - ubuntu	3m 36s
Tests: Ruby 2.6 - ubuntu	3m 35s
Tests: Ruby 2.3 - ubuntu	3m 40s
Tests: Ruby jruby - ubuntu	5m 51s
Tests: Ruby 3.3 - ubuntu	3m 20s
Tests: Ruby 3.0 - ubuntu	3m 21s
Tests: Ruby ruby - windows	4m 31s
Tests: Ruby truffleruby - ubuntu	4m 21s
Tests: Ruby 3.2 - ubuntu	3m 17s
Tests: Ruby 2.4 - ubuntu	3m 26s
Tests: Ruby 3.1 - ubuntu	3m 16s
Tests: Ruby 3.4 - ubuntu	3m 17s
	59m 57s

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