[lldb-dap][ext] Add script to only run extension unittests (#175010)

`vscode-test` requirest a GUI or something that emulates one. Use the
Mocha test library directly for unitest

Run unittests using `npm run unittest`

GitOrigin-RevId: 8ffb7f60da84447868b12c9f7a0e78722d80d1e2