Using mocha.report in .vscode-test.js results in "Test process exited unexpectedly" error on "Run Tests"

To Reproduce:

1. Follow https://code.visualstudio.com/api/get-started/your-first-extension to create a simple, unbundled TypeScript-based extension
2. Follow https://code.visualstudio.com/api/working-with-extensions/testing-extension to set up vscode-test
3. Select src/test/extension.test.ts and choose Run Tests from the context menu
4. This shows a successfully executing Sample test in the TEST RESULTS window
5. Edit .vscode-test.mjs by adding the following

   mocha: {
     reporter: "Spec"
   }
   

6. Select src/test/extension.test.tsagain and choose Run Tests from the context menu
7. This shows an error for Sample test

   Test process exited unexpectedly, view output

In particular, this masks any real test failures. All tests are shown as failures because of this.

Running with a configured mocha.reporter on the command-line (via pnpm test in my case) works as expected.

Versions:

• "@vscode/test-cli": "^0.0.10",
• "@vscode/test-electron": "^2.5.
• Version: 1.101.2
• Commit: 2901c5ac6db8a986a5666c3af51ff804d05af0d4
• Date: 2025-06-24T20:27:15.391Z
• Electron: 35.5.1
• ElectronBuildId: 11727614
• Chromium: 134.0.6998.205
• Node.js: 22.15.1
• V8: 13.4.114.21-electron.0
• OS: Linux x64 6.11.0-28-generic snap

Start by reproducing the issue with the sample unbundled TypeScript extension, editing .vscode-test.mjs to add the Mocha reporter and running Run Tests. Compare this with the working pnpm test command; done means configured reporters work in Run Tests without turning every test into an unexpected process failure.