Add tests for merge sort (#3715)

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

@@ -0,0 +1,33 @@
+package com.thealgorithms.strings;
+
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+public class ReverseStringRecursiveTest {
+    ReverseStringRecursive stringRecursive = new ReverseStringRecursive();
+
+    @Test
+    void shouldAcceptWhenEmptyStringIsPassed() {
+        String expected = "";
+        String reversed  = stringRecursive.reverse("");
+
+        assertEquals(expected,reversed);
+    }
+
+    @Test
+    void shouldAcceptNotWhenWhenSingleCharacterIsPassed() {
+        String expected = "a";
+        String reversed  = stringRecursive.reverse("a");
+
+        assertEquals(expected,reversed);
+    }
+
+    @Test
+    void shouldAcceptWhenStringIsPassed() {
+        String expected = "dlroWolleH";
+        String reversed  = stringRecursive.reverse("HelloWorld");
+
+        assertEquals(expected,reversed);
+    }
+}