Add unit tests for Correlation class

Added unit tests for the Correlation class to validate correlation calculations under various scenarios, including linear dependence and constant values.

Author: kvadrik

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

@@ -0,0 +1,48 @@
+package com.thealgorithms.maths;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.CsvSource;
+
+/**
+ * Test class for Correlation class
+ */
+public class CorrelationTest {
+
+    public static final double DELTA = 1e-9;
+
+    public void testCorrelationFirst() {
+        double[] x = {1, 2, 3, 4};
+        double[] y = {7, 1, 4, 9};
+        int n = 4;
+        assertEquals(0.3319700011, Correlation.correlation(x, y, n), DELTA);
+    }
+	
+    public void testCorrelationSecond() {
+        double[] x = {1, 2, 3, 4};
+        double[] y = {5, 0, 9, 2};
+        int n = 4;
+        assertEquals(0, Correlation.correlation(x, y, n), DELTA);
+    }        
+
+    public void testCorrelationConstant() {
+        double[] x = {1, 2, 3};
+        double[] y = {4, 4, 4};
+        int n = 3;
+        assertEquals(0, Correlation.correlation(x, y, n), DELTA);
+
+    public void testCorrelationLinearDependence() {
+        double[] x = {1, 2, 3, 4};
+        double[] y = {6, 8, 10, 12};
+        int n = 4;
+        assertEquals(1, Correlation.correlation(x, y, n), DELTA);
+    }
+
+    public void testCorrelationInverseLinearDependence() {
+        double[] x = {1, 2, 3, 4, 5};
+        double[] y = {18, 15, 12, 9, 6};
+        int n = 5;
+        assertEquals(-1, Correlation.correlation(x, y, n), DELTA);
+    }
+}