Faster tests

[Julow]

It's not reasonable to add as many tests as we'd need in the current heavy-weight test framework. Time and resources are the limitation, the CI currently runs in up to 2 hours.

To build a robust CLI tool, we need lightweight tests. We currently lack good test coverage.

Example of lightweight tests we'd need:

• The various expected behaviors (eg. when there's nothing to install: Fix crash when there's nothing to install #142)
• Cache migrations (it has many branches, Cache migration #137)
• Currently heavy tests that don't need to run with actual packages (reinstallation from cache, ocamlformat version detection)

Cram tests combined with a mock repository of quickly installing packages is something to investigate. A "dry-run" mode that avoids any call to Opam is also to consider.