shutdown executorService in benchmark after each

src/test/java/performance/PerformanceTest.java

@@ -1,5 +1,6 @@
 package performance;
 
+import org.junit.jupiter.api.AfterEach;
 import org.junit.jupiter.api.Assertions;
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
@@ -51,6 +52,11 @@ public class PerformanceTest {
         executorService = Executors.newFixedThreadPool(Runtime.getRuntime().availableProcessors());
     }
 
+    @AfterEach
+    void tearDown() {
+        executorService.shutdown();
+    }
+
     @Test
     void plainPerformanceTest() throws Exception {
         var orders = benchmark(numTraders, numOrders, pairs, PlainCurrencyExchange::new, null, false);