Simplify "Building for testing"

Now that rails/rails#46664 is merged, we can simplify the testing instructions.

Technically these instructions aren't true until Rails 7.1, so I'm happy to leave the PR open until then if that's preferred.

Author: ghiculescu

README.md

@@ -28,13 +28,8 @@ The `tailwindcss:build` is automatically attached to `assets:precompile`, so bef
 
 ### Building for testing
 
-The `tailwindcss:build` task is automatically attached to the `test:prepare` Rake task. The `test:prepare` task is run before some test tasks (e.g. `test:all` and `test:controllers`), but not before the bare `test` task.
+The `tailwindcss:build` task is automatically attached to the `test:prepare` Rake task. This task runs before test commands. If you run `bin/rails test` in your CI environment, your Tailwind output will be generated before tests run.
 
-If your tests need Tailwind assets in your CI environment, it's best to be explicit and run this command:
-
-```
-bin/rails test:prepare test
-```
 
 ### Update assets automatically