Add some testing

By: Erik Broes <erikbroes@grum.nl>

paper-api/src/test/java/org/bukkit/DifficultyTest.java

@@ -0,0 +1,15 @@
+package org.bukkit;
+
+import static org.hamcrest.CoreMatchers.is;
+import static org.junit.Assert.assertThat;
+
+import org.junit.Test;
+
+public class DifficultyTest {
+    @Test
+    public void getByValue() {
+        for (Difficulty difficulty : Difficulty.values()) {
+            assertThat(Difficulty.getByValue(difficulty.getValue()), is(difficulty));
+        }
+    }
+}