Add more end-2-end tests for Alloy language features

[thampiotr]

In recent release 1.7 we made some large refactors to the way that targets are handled. A lot of tests were added with the feature, but still some issues were found after the release which required bugfix patches.

This task is to add more tests for Alloy language features to reduce the risk of changes causing issues like this in the future. For example, we could make more extensive use of txtar-style tests that we currently have in Alloy.