Rewrite tests to be less brittle than golden file matching

[aiuto]

• Tests should be less brittle than exact file matching.
• It should be easy to rewrite them when vendoring in to your code base.

Towards the second point, we want to be able to minimize the number of places where we have to rewrite @rules_license to the //path/to/my/rules_license if we import the package into SCM. Test data should not be transformed, and the test itself might have a single constant within it (or injected to the test BUILD target) to provide the root of the tree.