Make butane/tests a separate crate

[jayvdb]

Removes build.rs script from butane crate, which did nothing except in development.

Related to #212

I suspect this might help with #212

[Electron100]

Do you see any other advantage to this, or just worth trying as a solution to #212?

[jayvdb]

Removing the build.rs is definitely a positive.

A negative is now an explicit Cargo.toml entry is needed for each test.

And there is a general "surprise" factor. e.g. we'll need to remember to do cargo test -p butane_tests instead of cargo test -p butane.
We could move butane_tests to the top level of the repo if that becomes a problem, likely when there are lots of new contributors who will be confused by butane/tests not using auto-discovery.

Also there are Rust infrastructure which run the tests of crates to check for potential problems with new language syntax or features, so removing the tests from butane is depriving such tools of our tests. Not a serious problem IMO.