Bug 1094151 - Write a test to install & launch a packaged app from the M...

...arketplace
Something like:
``` Python
class MarketplaceDev(Base):
    name = 'foobar'    # Marketplace Dev and Stage share the same name as the actual Marketplace
    # So we define the manifest_url instead
    manifest_url = 'https://marketplace.firefox.com/app/965bbfd7-936d-451d-bebf-fafdc7ce8d9e/manifest.webapp'

   def __init__(self, ...):
        ...
```

This would help us to avoid to copy and paste the manifest_url in each test using it.