add boilerplate for tests + move template to a new file

[oriongonza]

This slightly improves the boilerplate gen. I think that cargo-aoc users always want the two tests that come for free

[gobanos]

Hey, thanks for your contribution!
However the PR is not based on current state of the branch, and have a similar behavior to cargo-aoc/template/src/day.rs.tpl.
Would you rebase your PR, or close it if the changes are similar enough?

[oriongonza]

Done, this adds a bit more boilerplate for the tests, which almost always have the same structure.

[oriongonza]

I personally use a different template than what I provided here in this PR. It might be useful to provide a template at runtime.

cargo input -g --template src/codegen.rs.tpl