Add tests for the Web Labels webpack plugin

swh-web contains a custom webpack plugin enabling to generate a Web Labels table to state the licenses of all the Javascript files loaded by an application compiled with webpack.

This plugin lacks testing to ensure it will not break as new webpack versions wil be released.

A first bite on this was made by @vlorentz in !75 (closed).

Other type of tests could consist in compiling a sample webpack application and check the Web Labels table is correctly generated with the expected content.

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Migrated from T3122 (view on Phabricator)