Add unit tests for SimpleSubCipher (#3688)

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

@@ -0,0 +1,37 @@
+package com.thealgorithms.ciphers;
+
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+class SimpleSubCipherTest {
+
+    SimpleSubCipher simpleSubCipher = new SimpleSubCipher();
+
+    @Test
+    void simpleSubCipherEncryptTest() {
+        // given
+        String text = "defend the east wall of the castle";
+        String cipherSmall = "phqgiumeaylnofdxjkrcvstzwb";
+
+        // when
+        String cipherText = simpleSubCipher.encode(text, cipherSmall);
+
+        // then
+        assertEquals("giuifg cei iprc tpnn du cei qprcni", cipherText);
+    }
+
+    @Test
+    void simpleSubCipherDecryptTest() {
+        // given
+        String encryptedText = "giuifg cei iprc tpnn du cei qprcni";
+        String cipherSmall = "phqgiumeaylnofdxjkrcvstzwb";
+
+        // when
+        String decryptedText = simpleSubCipher.decode(encryptedText, cipherSmall);
+
+        // then
+        assertEquals("defend the east wall of the castle", decryptedText);
+    }
+
+}