This PR is a WIP that aims to address the first 3 steps in #1833.
This is in anticipation of using `forc test` to support unit testing
(rather than Rust integration testing) #1832.
The `forc test` command remains for now, but outputs a message
explaining that the command is now reserved for unit testing and links
to the issues above.
## TODO
- [x] Create a new `sway-test-rs` repo or similar that can be used as a
`cargo generate` template.
- [x] Update Rust integration testing docs in the Sway book to describe
how to use the `cargo generate` command to easily add Sway integration
testing to an existing Rust project.
## Follow-up
- Create a `forc-test-rs` crate that re-exports and extends `fuels` with
useful `forc` functionality for integration testing (e.g. re-exporting
`forc_pkg::build` to ensure sway code is built and available under
`out/` at the start of testing).
