Make it easy to test templates

[pledbrook]

The integration test framework should be made available for template authors so that they can easily write tests.

[ingorichter]

+1

[pledbrook]

There are a couple of unique challenges here. The Gradle plugin already has the ability to install the templates in the local Lazybones cache, so that's not a problem. But we need some way of sharing AbstractFunctionalSpec between the integration tests of the lazybones-app project and the lazybones-gradle-plugin package.

We also need a way to specify a Lazybones installation to use for each test run. What if a template developer wants to run the tests against multiple versions of Lazybones? Perhaps different Lazybones versions will result in different behaviour, such as one version resulting in a LazybonesScriptException due to detected incompatibilities.