Add test case for DecimalToHexaDecimal (#3697)

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

@@ -0,0 +1,14 @@
+package com.thealgorithms.conversions;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+import org.junit.jupiter.api.Test;
+
+public class DecimalToHexaDecimalTest {
+
+    @Test
+    public void testDecimalToHexaDecimal() {
+        assertEquals("000000be", DecimalToHexaDecimal.decToHex(190));
+        assertEquals("00000708", DecimalToHexaDecimal.decToHex(1800));
+    }
+}