Fix UI unit tests

[framebassman]

Welcome!

• Yes, I've searched similar issues on GitHub and didn't find any.
• Yes, I've searched similar issues on the Traefik community forum and didn't find any.

What did you expect to see?

Motivation
As a Traefik Frontend Developer I want to unit test the changes in webui project

I got the following output when I try to run unit tests for webui project

xxxxxxxxxxxxxxx :: personal/traefik/webui » yarn run test-unit
yarn run v1.22.19
$ mocha-webpack --mode=production './src/**/*.spec.js'
/bin/sh: 1: mocha-webpack: not found
error Command failed with exit code 127.
info Visit https://yarnpkg.com/en/docs/cli/run for documentation about this command.

[framebassman]

The ticket should be resolved in #10665