MiniCssExtractPlugin tests

Based on docs: https://webpack.js.org/plugins/mini-css-extract-plugin/#extracting-css-based-on-entry

Looks like there is a bug - plugin overwrites some css files and throws errors/warnings

Reproduce

1. Build

$ npm i && npm run build

2. Check the dist folder.

UPD

Some answers provided in this issue: webpack/mini-css-extract-plugin#804