bug: not all packages have a test/build script

[regevbr]

We currently assume that there is an npm script called "test", but that is not always the case (see https://github.com/yarnpkg/berry/blob/master/package.json for example)
We need to think of a way to realize the test script if one doesn't exist (and to verify its existence before hand)

[regevbr]

Maybe we can show the user the list of available scripts and let him choose which one to use.
In that case I think we need to add this as configuration per package, and also persist the script name in the cache

[regevbr]

Same goes for the build/compile script