Add unit tests for Vigenere cipher (#3666)

src/test/java/com/thealgorithms/ciphers/VigenereTest.java

@@ -0,0 +1,37 @@
+package com.thealgorithms.ciphers;
+
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+class VigenereTest {
+
+    Vigenere vigenere = new Vigenere();
+
+    @Test
+    void vigenereEncryptTest() {
+        // given
+        String text = "Hello World!";
+        String key = "suchsecret";
+
+        // when
+        String cipherText = vigenere.encrypt(text, key);
+
+        // then
+        assertEquals("Zynsg Yfvev!", cipherText);
+    }
+
+    @Test
+    void vigenereDecryptTest() {
+        // given
+        String encryptedText = "Zynsg Yfvev!";
+        String key = "suchsecret";
+
+        // when
+        String decryptedText = vigenere.decrypt(encryptedText, key);
+
+        // then
+        assertEquals("Hello World!", decryptedText);
+    }
+
+}