Currently the build/test fails with the following output:

```
Test webextension-polyfill bundled with webpack
===============================================
[webpack-cli] Unknown argument: --output
[webpack-cli] Unknown argument: /tmp/webpack-bundle.js
? Which flags do you want to use? …
✔ --entry: The entry point(s) of your application e.g. ./src/main.js
✔ --config: Provide path to a webpack configuration file e.g. ./webpack.config.js
✔ --color: Enable/Disable colors on console
✔ --merge: Merge two or more configurations using webpack-merge e.g. -c ./webpack.config.js -c ./webpack.test.config.js --merge
✔ --progress: Print compilation progress during build
✔ --help: Outputs list of supported flags
✔ --output-path: Output location of the file generated by webpack e.g. ./dist/
```

Regressed by:
webpack/webpack-cli#1855
webpack/webpack-cli@2f36b9d

`--output` has been renamed to `--output-path` (`-o` in both cases), but
stopped supporting file names.  To output to a specific absolute path,
`--output-path` and `--output-filename` must both be specified, with the
directory in the former and the filename in the latter.