Run tests against installed package

[bryphe]

We seem some failures due to the unpacking / packing logic (for example, symlinks not being hydrated correctly).

We should run our test suite against the install package, too. We could do an additional run after we've packed and specify the esy-bash root location via an environment variable.

[bryphe]

An example of a failure that would be caught by tests like these is: #33

[bryphe]

This a little awkward to do in AppVeyor at the moment, because we publish in our deploy_script block - we could potentially move that logic to the build step, and run the tests after. This is still awkward because we'd be running tests after potentially publishing the package.

Alternatively, we could:

• npm pack
• npm install the tgz to a separate directory
• run tests against that separate directory
• delete the tgz

[andreypopp]

Installing with npm pack && npm install seems like the way to go.