Clarify test package use on 'An introduction to unit testing' page

[eliasyishak]

Page URL

https://docs.flutter.dev/cookbook/testing/unit/introduction/

Page source

https://github.com/flutter/website/tree/main/src/cookbook/testing/unit/introduction.md

Describe the problem

Per this issue, we are now recommending users that running flutter test to only use the flutter_test dependency in their pubspec.yaml file. However, in this link, for unit tests, it gives the user the option to either use the test or flutter_test depedency.

Expected fix

We should only recommend to use the flutter_test dependency to ensure that we don't have any version solving issues for the flutter test command specifically.

Additional context

Because flutter_test depends on pinned versions of test_api, we can ensure that we won't have any version mismatches between the app pubspec and the flutter tool pubspec. More details can be found in the below issue:

• https://github.com/flutter/flutter/issues/91107

I also have a PR to ensure we check for this dependency in the app pubspec:

• https://github.com/flutter/flutter/pull/139498

I would like to fix this problem.

• [ ] I will try and fix this problem on docs.flutter.dev.