Module name mappings not good in Yeoman Volto generator for volto-slate

[silviubogan]

After overcoming #44 as I said in that issue, I get these sort of fails from unit tests (part of the output of $ yarn test and there is no script $ yarn test-addon in the generated package.json file that I can run):

volto-slate/editor cannot be imported because it is not seen as an existing file or directory.

I need this working because I would like to use the latest Jest version for unit tests in volto-slate but when I updated the version of Jest here, I came over this issue for which, to make it progress, I would need an isolated testcase and I thought to make it based on Yeoman.