Improve integration tests setup

[frangio]

Our integration tests are not satisfactory in catching several types of errors because they are not set up isolated, but sharing dependencies with the rest of the monorepo.

I think the correct way to set them up would be to use npm pack and store the .tgz as a build artifact, then pick it up from a different job and install it, which mirrors a real scenario much better. A bit of a challenge is to make it also work locally, although in that case it doesn't need to be as robust, as long as the CI catches errors later.

Because this is a monorepo with multiple packages, it might not be enough to use the .tgz since they depend on one another. What the web3.js project is doing is quite cool: they set up a private npm registry in the CI where they put all the package candidates, and the tests install from there. (web3/web3.js#3157)