Add ElGamalCipher unit tests

Author: polasisubash

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

@@ -0,0 +1,41 @@
+package com.thealgorithms.ciphers;
+
+import static org.junit.jupiter.api.Assertions.*;
+import org.junit.jupiter.api.Test;
+
+public class ElGamalCipherTest {
+
+    @Test
+    void testEncryptThenDecrypt_returnsOriginal() {
+        ElGamalCipher elg = new ElGamalCipher();
+
+        String original = "HelloElGamal123";
+        String encrypted = elg.encrypt(original);
+        assertNotNull(encrypted);
+
+        String decrypted = elg.decrypt(encrypted);
+        assertEquals(original, decrypted);
+    }
+
+    @Test
+    void testEncryptedIsDifferentFromOriginal() {
+        ElGamalCipher elg = new ElGamalCipher();
+        String encrypted = elg.encrypt("TestData");
+        assertNotEquals("TestData", encrypted);
+    }
+
+    @Test
+    void testEmptyString() {
+        ElGamalCipher elg = new ElGamalCipher();
+        String encrypted = elg.encrypt("");
+        String decrypted = elg.decrypt(encrypted);
+        assertEquals("", decrypted);
+    }
+
+    @Test
+    void testNullInputs() {
+        ElGamalCipher elg = new ElGamalCipher();
+        assertThrows(IllegalArgumentException.class, () -> elg.encrypt(null));
+        assertThrows(IllegalArgumentException.class, () -> elg.decrypt(null));
+    }
+}