refactor(scripts): Unify build and test scripts under the scripts directory.

It might only be a problem for me, but it slightly bothers me that there are three scripts in two different locations, and with two different name conventions.

This PR moves `build` and `test.sh` to the `scripts` directory. It also drops the `.sh` from `test.sh` to be consistent with the other scripts.
It also adds a mention in the contributing guide about how to run the tests.