Implement unit tests for GCDUsingEuclid

Add unit tests for GCDUsingEuclid class covering positive, zero, and negative numbers.

Author: Swati89923

src/test/java/com/thealgorithms/maths/GCDUsingEuclidTest.java

@@ -0,0 +1,25 @@
+package com.thealgorithms.maths;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import org.junit.jupiter.api.Test;
+
+class GCDUsingEuclidTest {
+
+    @Test
+    void testPositiveNumbers() {
+        assertEquals(6, GCDUsingEuclid.gcd(54, 24));
+        assertEquals(1, GCDUsingEuclid.gcd(17, 13));
+    }
+
+    @Test
+    void testWithZero() {
+        assertEquals(5, GCDUsingEuclid.gcd(0, 5));
+        assertEquals(7, GCDUsingEuclid.gcd(7, 0));
+    }
+
+    @Test
+    void testNegativeNumbers() {
+        assertEquals(4, GCDUsingEuclid.gcd(-8, 12));
+        assertEquals(9, GCDUsingEuclid.gcd(-9, -18));
+    }
+}