`ThreadPoolExecutor#kill` will `wait_for_termination` in JRuby; ensure `TimerSet` timer thread shuts down cleanly #638

Job	Run time
Test isolated (ruby)	3m 42s
Test isolated (2.3)	3m 55s
Test without C extension	3m 17s
Tests: Ruby 3.2 - ubuntu	3m 23s
Tests: Ruby 2.6 - ubuntu	3m 40s
Tests: Ruby 2.3 - ubuntu	3m 32s
Tests: Ruby ruby - windows	4m 25s
Tests: Ruby 2.7 - ubuntu	3m 32s
Tests: Ruby 2.4 - ubuntu	3m 32s
Tests: Ruby 3.4 - ubuntu	3m 18s
Tests: Ruby 3.1 - ubuntu	3m 17s
Tests: Ruby 3.0 - ubuntu	3m 19s
Tests: Ruby 2.5 - ubuntu	3m 32s
Tests: Ruby 3.3 - ubuntu	3m 24s
Tests: Ruby jruby - ubuntu	10m 16s
Tests: Ruby truffleruby - ubuntu	4m 21s
Test without C extension	3m 17s
Test isolated (2.3)	3m 55s
Tests: Ruby 2.7 - ubuntu	3m 32s
Tests: Ruby 2.4 - ubuntu	3m 32s
Tests: Ruby 2.5 - ubuntu	3m 32s
Tests: Ruby 3.2 - ubuntu	3m 23s
Tests: Ruby truffleruby - ubuntu	4m 18s
Tests: Ruby jruby - ubuntu	10m 15s
Tests: Ruby 2.6 - ubuntu	3m 40s
Tests: Ruby 3.0 - ubuntu	3m 19s
Test isolated (ruby)	3m 42s
Tests: Ruby 2.3 - ubuntu	3m 32s
Tests: Ruby 3.3 - ubuntu	3m 24s
Tests: Ruby 3.4 - ubuntu	3m 18s
Tests: Ruby 3.1 - ubuntu	3m 17s
Tests: Ruby ruby - windows	4m 25s
	2h 8m 46s

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