docs: update swayfmt contributing (FuelLabs#3936)

prompts the contributor within the `swayfmt` contributing guide to run
the test suite as well as use the test suite as a guide on how to write
tests.

swayfmt/CONTRIBUTING.md

@@ -30,7 +30,21 @@ cargo build --path ~/sway/forc-plugins/forc-fmt/Cargo.toml && mv ~/sway/target/d
 
 ## Testing
 
-Be sure to have `forc` installed then move into a Sway project directory and execute the binary:
+You should test your changes both using the test suite as well as manually.
+
+Before you make changes, let's ensure that the test suite runs fine:
+
+```sh
+# from the root
+cd swayfmt && cargo test
+```
+
+`swayfmt` has an extensive test suite that should pass both locally and within the CI to ensure reliability. This is used
+to ensure that there isn't regression introduced along with new changes. If your changes include fixing bugs or adding
+a new feature, please also include new tests accompanying your PR where possible. You may look at [formatter/mod.rs](https://github.com/FuelLabs/sway/blob/master/swayfmt/src/formatter/mod.rs)
+for examples on how you may test your changes.
+
+To manually test the formatter, be sure to have `forc` installed then move into a Sway project directory and execute the binary:
 
 ```sh
 forc fmt