Change test to flutter_test on the 'An introduction to unit testing' page

[hk6an6]

Page URL: https://docs.flutter.dev/cookbook/testing/unit/introduction
Page source: https://github.com/flutter/website/tree/master/src/docs/cookbook/testing/unit/introduction.md

Description of issue:

The lines 35 to 45 are misleading and will not yield working code. The documentation introduces 2 problems that will be confusing for beginners:

1. When adding dependencies, the name of the file to modify is missing. It should be pubsec.yaml.

2. When suggesting what to put in the file the documentation in line 43 recommends test: <latest_version> - which will not compile. Instead the entry needs to look like this:

# in pubsepc.yaml:
dev_dependencies:
  flutter_test:
    sdk: flutter

[arps18]

Hey @sfshaza2 can I work on the issue?

[sfshaza2]

Yup! But does this page use code excerpts? If so, that requires special handling.

[atsansone]

This should replace:

• Revise 'An introduction to unit testing' page to focus on flutter_test rather than test package. #2837
• Clarify relationship between flutter_test and test packages on 'An introduction to unit testing' page #5371

Change lines 18 to 19. To clarify, the page should state:

You need the [`flutter_test`](https://pub.dev/packages/test).
It contains the [`test`](https://api.flutter.dev/flutter/flutter_test/flutter_test-library.html)
package and provides utilities for testing widgets.

Replace test with flutter_test throughout.

[atsansone]

Work on the following at the same time:

• Explain, link group function on 'An introduction to unit testing' page #5761
• Clarify that dependency is set in pubspec.yaml on 'An introduction to unit testing' page #3146

[brianegan]

Sure, no problems here. My goal was to indicate that unit tests are separate from flutter tests, and that there are many cases where you may want to test business logic independent of Flutter. Therefore, using raw test package is better IMO so you do not accidentally introduce Flutter dependencies in your business logic layer (which should operate independently of any GUI -- at the time, AngularDart was still officially supported and I was working on projects where sharing logic between Flutter and AngularDart was a requirement).

I don't feel strongly, however, if you think this is a better direction.

[brianegan]

Please also see the historical context here: a786092

#2497

Again, these recipes are over 4 years old (maybe closer to 5-6 years old). So these decisions should be revisited.

[atsansone]

Fixed in #8950