add tests for methods required by the password manager interface

src/test/java/PasswordManagerTest.java

@@ -1,11 +1,30 @@
+import org.junit.jupiter.api.BeforeAll;
 import org.junit.jupiter.api.Test;
 
 import static org.junit.jupiter.api.Assertions.*;
 
+
 class PasswordManagerTest {
 
+    static PasswordManager pm;
+
+    @BeforeAll
+    static void init() {
+        pm = new PasswordManager();
+    }
+
     @Test
     void constructor() {
-        PasswordManager pm = new PasswordManager();
+        assertInstanceOf(PasswordManager.class, pm);
+    }
+
+    @Test
+    void listVaults() {
+        pm.listVaults();
+    }
+
+    @Test
+    void openVault() {
+        pm.openVault();
     }
 }