Improve testing of Lake examples

[david-christiansen]

Right now, only the parsing of Lake configuration examples is tested. This risks missing later validation steps, such as in #272.

Ideally, Lake snippets would be tested more thoroughly. Some of the validation steps require actually running Lake with real packages, however - the error thrown in #272 happens while actually fetching the package from Reservoir. A proper test setup would need to run a small Reservoir-compatible server and run Lake in a project in a temporary directory.