Revise 'An introduction to unit testing' page to focus on flutter_test rather than test package.

[feinstein]

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 beginning of the page says:

The test package provides the core framework for writing unit tests, and the flutter_test package provides additional utilities for testing widgets.

But the rest of page fails to expose what's the difference between the test and flutter_test packages. After a short search I could find that flutter_test is built on top of the test package, which makes me wonder, why this Flutter cookbook suggest to add only a test package dependency then?

[sfshaza2]

@brianegan, can you comment?

[atsansone]

This issue should be handled at the same time as #5371.

[atsansone]

This is a duplicate of #5372. Closing.