[lumino] mock DragEvent in test setup

'@lumino/dragdrop' consumes DragEvent already at loading time.
Therefore the DragEvent mock must already be available before executing
the tests.

Adds a new private '@theia/test-setup' dev package which is consumed in
the mocha config to mock 'DragEvent' before loading and compiling
tests.

Contributed on behalf of STMicroelectronics

configs/mocharc.yml

@@ -1,6 +1,7 @@
 require:
   - 'ignore-styles'
   - 'reflect-metadata/Reflect'
+  - '@theia/test-setup'
 reporter: 'spec'
 watch-files:
     - '**/*.js'

dev-packages/private-test-setup/README.md

@@ -0,0 +1,32 @@
+<div align='center'>
+
+<br />
+
+<img src='https://raw.githubusercontent.com/eclipse-theia/theia/master/logo/theia.svg?sanitize=true' alt='theia-ext-logo' width='100px' />
+
+<h2>ECLIPSE THEIA - TEST SETUP</h2>
+
+<hr />
+
+</div>
+
+## Description
+
+The `@theia/test-setup` contributes a setup script for mocha executed tests in Theia.
+This setup script is executed before any test file is loaded or compiled.
+This is for example useful for globals which must exist before dependencies are loaded.
+
+## Additional Information
+
+- [Theia - GitHub](https://github.com/eclipse-theia/theia)
+- [Theia - Website](https://theia-ide.org/)
+
+## License
+
+-   [Eclipse Public License 2.0](http://www.eclipse.org/legal/epl-2.0/)
+-   [一 (Secondary) GNU General Public License, version 2 with the GNU Classpath Exception](https://projects.eclipse.org/license/secondary-gpl-2.0-cp)
+
+## Trademark
+
+"Theia" is a trademark of the Eclipse Foundation
+https://www.eclipse.org/theia

dev-packages/private-test-setup/package.json

@@ -0,0 +1,7 @@
+{
+  "private": true,
+  "name": "@theia/test-setup",
+  "version": "1.56.0",
+  "description": "Custom setup for mocha tests",
+  "main": "test-setup.js"
+}

dev-packages/private-test-setup/test-setup.js

@@ -0,0 +1,18 @@
+// *****************************************************************************
+// Copyright (C) 2024 STMicroelectronics and others.
+//
+// This program and the accompanying materials are made available under the
+// terms of the Eclipse Public License v. 2.0 which is available at
+// http://www.eclipse.org/legal/epl-2.0.
+//
+// This Source Code may also be made available under the following Secondary
+// Licenses when the conditions for such availability set forth in the Eclipse
+// Public License v. 2.0 are satisfied: GNU General Public License, version 2
+// with the GNU Classpath Exception which is available at
+// https://www.gnu.org/software/classpath/license.html.
+//
+// SPDX-License-Identifier: EPL-2.0 OR GPL-2.0-only WITH Classpath-exception-2.0
+// *****************************************************************************
+
+// Mock DragEvent as '@lumino/dragdrop' already requires it at require time
+global.DragEvent = class DragEvent { };