Add tests for GCD and PrimeCheck (#3062)

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

@@ -0,0 +1,41 @@
+package com.thealgorithms.maths;
+
+import org.junit.jupiter.api.Assertions;
+import org.junit.jupiter.api.Test;
+
+public class GCDTest {
+    @Test
+    void test1() {
+        Assertions.assertThrows(ArithmeticException.class, () -> GCD.gcd(-1,0));
+    }
+
+    @Test
+    void test2() {
+        Assertions.assertThrows(ArithmeticException.class, () -> GCD.gcd(10, -2));
+    }
+
+    @Test
+    void test3() {
+        Assertions.assertThrows(ArithmeticException.class, () -> GCD.gcd(-5, -3));
+    }
+
+    @Test
+    void test4() {
+        Assertions.assertEquals(GCD.gcd(0, 2), 2);
+    }
+
+    @Test
+    void test5() {
+        Assertions.assertEquals(GCD.gcd(10, 0), 10);
+    }
+
+    @Test
+    void test6() {
+        Assertions.assertEquals(GCD.gcd(1, 0), 1);
+    }
+
+    @Test
+    void test7() {
+        Assertions.assertEquals(GCD.gcd(9, 6), 3);
+    }
+}