Add mypy_test rule

Depending on your project, you may not want to run lints like this as an aspect, since it potentially impacts developer's tight loop iterating on some python. I think it would be nice if this ruleset also provided a test rule that could be used to run mypy. I imagine it would be possible to still use aspects from that test to share the mypy cache with deps, but I haven't tried yet.

Start by reading the existing mypy aspect and the repository's Bazel rule and test patterns. Determine how a test rule should run mypy without forcing aspect-based linting, and whether sharing the mypy cache with dependencies is feasible; done means the ruleset exposes a usable test rule with documented behavior and passing tests.

Written by the indexing model from the issue text.