Add test case for HexToOct (#3675)

src/test/java/com/thealgorithms/conversions/HexToOctTest.java

@@ -0,0 +1,14 @@
+package com.thealgorithms.conversions;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+import org.junit.jupiter.api.Test;
+
+public class HexToOctTest {
+
+    @Test
+    public void testHexToOct() {
+        assertEquals(110, HexToOct.decimal2octal(HexToOct.hex2decimal("48")));
+        assertEquals(255, HexToOct.decimal2octal(HexToOct.hex2decimal("AD")));
+    }
+}