Add ReverseStringTest (#3607)

Co-authored-by: jtuong <Rofleveryday1?>

src/test/java/com/thealgorithms/strings/ReverseStringTest.java

@@ -0,0 +1,31 @@
+package com.thealgorithms.strings;
+
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+public class ReverseStringTest {
+
+    @Test
+    public void ReverseStringTest() {
+        String input1 = "Hello World";
+        String input2 = "helloworld";
+        String input3 = "123456789";
+        String input4 = "";
+
+        String expectedOutput1 = "dlroW olleH";
+        String expectedOutput2 = "dlrowolleh";
+        String expectedOutput3 = "987654321";
+        String expectedOutput4 = "";
+
+        assertEquals(ReverseString.reverse(input1), expectedOutput1);
+        assertEquals(ReverseString.reverse(input2), expectedOutput2);
+        assertEquals(ReverseString.reverse(input3), expectedOutput3);
+        assertEquals(ReverseString.reverse(input4), expectedOutput4);
+
+        assertEquals(ReverseString.reverse2(input1), expectedOutput1);
+        assertEquals(ReverseString.reverse2(input2), expectedOutput2);
+        assertEquals(ReverseString.reverse2(input3), expectedOutput3);
+        assertEquals(ReverseString.reverse2(input4), expectedOutput4);
+    }
+}