tests: Update tests to latest Server changes

worldedit-bukkit/src/test/java/com/fastasyncworldedit/util/StubServer.java

@@ -993,6 +993,11 @@ public class StubServer implements Server {
         return 0;
     }
 
+    @Override
+    public @NotNull File getPluginsFolder() {
+        return null;
+    }
+
     @Override
     public boolean isStopping() {
         return false;