fix: improve coverage on publisher-subscriber design pattern (iluwatar#2898)

Author: sanurah

publish-subscribe/src/test/java/com/iluwatar/publish/subscribe/AppTest.java

@@ -0,0 +1,13 @@
+package com.iluwatar.publish.subscribe;
+
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
+
+import org.junit.jupiter.api.Test;
+
+public class AppTest {
+
+  @Test
+  void shouldExecuteApplicationWithoutException() {
+    assertDoesNotThrow(() -> App.main(new String[]{}));
+  }
+}