Add tests for --migrate

As seen in #185 the entire --migrate workflow is untested, and thus a breakage went by unnoticed.
Unfortunately #186 demonstrates that adding a test is not straightforward, and ideally we'd have a fast unit test that doesn't send any HTTP requests to GitHub.

Start by tracing the --migrate workflow in the Bazelisk repository and read issues #185 and #186 for the observed breakage and testing constraints. Identify the existing workflow tests or entry points, then determine how a fast unit test can cover --migrate without sending HTTP requests to GitHub; done means the workflow is covered and the test remains network-free.

Written by the indexing model from the issue text.