Added a test for ThreadPool #4572

dbms/tests/queries/0_stateless/00913_many_threads.reference

+1
+1500
+0
+Ok.

dbms/tests/queries/0_stateless/00913_many_threads.sql

+-- This test creates many threads to test a case when ThreadPool will remove some threads from pool after job is done.
+SET max_block_size = 1, min_insert_block_size_rows = 0, min_insert_block_size_bytes = 0;
+
+CREATE TEMPORARY TABLE t (x UInt64);
+INSERT INTO t SELECT * FROM system.numbers LIMIT 1500;
+
+SELECT DISTINCT blockSize() FROM t;
+
+SET max_threads = 1500;
+
+SELECT count() FROM t;
+SELECT sum(sleep(0.1)) FROM t; -- All threads have time to be created.
+SELECT 'Ok.';