Did you know, that you just go into your launch config, put your cursor after or between your other configs and pressctrl -space to get a current, valid mocha config auto-generated?

Which works perfectly fine for me. Including stopping at breakpoints. ( I also had a prior, now outdated one, that did no longer for various setting-related reasons. )

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As of VSCode 1.21.1 (March 2018) this yields:

{
  "version": "0.2.0",
  "configurations": [
    {
      "name": "Mocha (Test single file)",
      "type": "node",
      "request": "launch",
      "runtimeArgs": [
        "${workspaceRoot}/node_modules/.bin/mocha",
        "--inspect-brk",
        "${relativeFile}",
      ],
      "console": "integratedTerminal",
      "internalConsoleOptions": "neverOpen",
      "port": 9229
    }
}

On a side-note: debug-brk is deprectated (for anyone with Node >= Version 8 at least).