Dependencies can leak between scenarios

If you have a scenario with no `npm` section at all, it *seems* to work, but it silently uses the NPM deps from the previous scenario.

Reproduction:

1. Clone and yarn install https://github.com/ef4/--try-multi-cli
2. `ember try:each` should show a failure.
3. Uncomment the section in ember-try.js and you will no longer get a failure.

(I created this test app initially because I was trying to confirm that ember-try can really swap the ember-cli version in use -- it can, thanks!)

This seems related to https://github.com/ember-cli/ember-try/issues/81 but not identical to it. That older issue seems to imply that this case would be an explicit error, whereas now it seems to be a tolerated-but-faulty.