jsii: Add "install" integration tests

[eladb]

To avoid issues like #313, we should include some integration tests in our pipeline that "npm installs" jsii from tarballs. This can be achieved either by "npm install js/*.tgz" or by using verdaccio.

[sam-goodwin]

I think we can re-use a bunch of the existing tests - add a JSII 'target' (or something) environment variable to switch between behavior. Saves re-doing veradccio and less boilerplate such as installing, checking canary mode, etc.

[eladb]

Can we extract the verdaccio stuff to a scripts that’s reusable between Jsii and cdk tests?